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When are Tires unsafe
Assuming even wear the tires must be replaced when 2/32" of tread remains. That is when the tread-wear bars start to appear horizontally across the tread.
Here in rainy western Oregon I replace our tires when they wear to 4/32". Hydroplaning is nasty business and your tread has to have sufficient depth to allow water to drain away. In a dryer climate I may go to 3/32", but I think 2/32" is unsafe in any climate. If the edges are worn enough to prevent water from draining out the sides of your tires hydroplaning risk increases a lot.
The classic explanation for tires wearing on the edges is underinflation. That sure can cause it, but there are other reasons. It could it caused by poor wheel alignment, weak shocks, mechanical problems in the front end, and even suspension design or tire construction. If the wear is sort of scalloped or cupped (not smoothly worn around the circumference of the tire) I would lean toward out-of-balance, weak shocks, or front-end problems.
If both inner and outer edges are worn smoothly around the circumference it would probably be underinflation. If the inner and outer edges are wearing differently it would generally be caused by alignment problems or suspension design. The old Ford I-Beam comes to mind - those suspensions would chew up tires like nobodys business. Vehicles with short control arms tend to wear edges faster than those with longer control arms.
And sometimes some brands just wear faster than others, and sometimes they do it in odd ways. New tires generally have anywhere from 9 to 13/32" of tread on them. Obviously the thicker the tread when new the longer they will last. Thick tread has a downside in that those tires tend to be a bit noisier and sing more at highway speeds. For this reason (and cost of course) manufacturers often put tires with lesser tread depth on new vehicles.
A tread depth gauge is a cheap item to have, and it's good to go out and measure your tread every couple of months to see if the tread is wearing evenly. You can catch problems early this way and save some bucks.
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When are Tires unsafe
It's easy for me to spend your money, but yes I would recommend replacement. People may throw stones at me for saying this, but I generally ignore the manufacturer tire pressure recommendations. If I am unsure about which pressure is best for any particular application I start with the maximum sidewall pressure. If the ride causes the fillings to fall out or your teeth you can back it off some, but keep it high. As you drive use a treadwear gauge every month or two to make sure they're wearing evenly. If they wear too much in the middle you can back off the pressure a few lbs, or if they wear too much on the edges you should bump it up even more.
As you indicated, manufacturers often specify a low tire pressure to give a smooth ride but that doesn't maximize tread life. I believe one factor in the Ford/Firestone Explorer tire fiasco was that Ford had specified too low a pressure. Low pressures increase heat build-up which leads to tread separation.
BTW, Michelin makes a fine tire but I discovered from my years in the tire shop that they seem to use less rubber per tire than nearly any other manufacturer. The inner liner of their tires is VERY thin - you need to be very careful when patching a michelin from the inside. This sounds bad, but less mass means cooler running and excess heat really shortens the life of a tire. For my money, though, on any vehicle which is going to see a hard off-road life or spend a lot of time on gravel roads I'd rather have MASS.
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When are Tires unsafe
Dennis,
that's quite interesting. I know Michelin makes a top-notch tire for the road but I'm not sure how good they are for off road or gravel road situations. We didn't see a lot of those applications at our shop. The guys who bought the big and beefy off-road tires never seemed to actually go off road.
It's really a matter of picking the right tire for the job, and avoiding marketing hype. For example, I really like to see a cross section of the tire I am purchasing. Why? Well in the 80's Goodyear sold a Polysteel steel belted radial. They sold quite well but seemed to have a lot of road hazard failures. One slow day we cut one open and found that the steel belt was about 2" wide covering the very center of the tread. The tire footprint was about 9-10" and only 20% of it was protected by the steel belt!
More plies is better for impact resistance and load carrying, but 8/10/12-ply tires will wear out faster at highway speeds than a 4 ply or 4 ply rated tire due to the extra heat. Nylon cord is the strongest and stretches the least but for passenger cars it is rough riding when cold (they take on a flat spot when sitting overnight and it doesn't go away until they warm up) so polyester is popular among passenger car tires.
We have to drive up a 5 mile rough gravel logging road to get to our house. When I tire shop I look for strength, traction, and impact resistance (nylon and steel belts rule!) and let the tire-wear chips fall where they may. I really hate changing tires on a narrow gravel road with logging trucks barreling by at breakneck speeds.
If I were still in the business it would be interesting to use one of those point-and-shoot infrared thermometers to measure tire temps as people drive in. I'm sure you could tell a lot about tire construction, alignment, and inflation by measuring at several points across the tread. That's how the racing guys do it.
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When are Tires unsafe
I'm reluctant to join a community weight-loss effort because my weight it coming off SO SLOW with Atkins. I've lost maybe 6 lbs in two months. Low fat diets seem to work much better for me, so I may go that route instead.
To tie it back into this thread, I can tell you when I was busting 40-50 tires/day I was in a LOT better shape. Physical labor has it's benefits 
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When are Tires unsafe
Thanks for the engineering perspective. I was thinking the additional thickness would have an insulative effect where the peak temperature might be a bit higher and the exposure period would be a bit longer. Maybe not.
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When are Tires unsafe
Speaking of Firestone, I know they CAN make a good tire the question is which ones are good?
Showing my age here, the Firestone 500 was a debacle pure and simple. Firestone screwed up and they know it. They replaced the 500 with the 721. The 721 was not a very reliable tire (occasional tread separations, belt slippages, etc), but at least the tread didn't go flying off whenever you hit 60 mph like it did on the 500. IIRC, the 721 was the top of the line Firestone passenger tire at the time.
The tire company I worked for sold a tire called the Gas Stretcher. This tire was made by Firestone and it was an incredibly good tire - very long life and I don't recall a single one coming back in for a workmanship claim. The Gas Stretcher was a far better tire than Firestone's best 721, so I have to wonder what they're thinking when they produce better tires for other companies than they do for themselves.
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