crutschow has got it right. I work for a semiconductor manufacturer and come across this problem quite often. Commercial grade parts are exactly the same silicon (and mostly the same packaging) as military grade parts. However military grade parts are tested (and thus guaranteed) to work over a wider temperature range.
Now for the lifetime... there used to be a rule that you should never use a part above 100 degC or its lifetime will be greatly reduced. This was the case back in the 90's. That is not really the case now (at least not with the devices my company make). Failure rates are so low that, even at high temperatures, the part's lifetime can run into thousands of years (statistically). The manufacturer will not stand by any guarantees and you are definitely on your own, but it should still work. You need to look at the MTBF data for the part and extrapolate using the Arrenhius equation. That will tell you, statistically, how long the part should last. However, this is only applicable to the silicon. it does not allow for stress on the solder joints as a result of temperature cycling. These will go open circuit long before the silicon gives up.
Higher temperatures also causes the package to deform placing stresses on the die which means some of the characteristics might wander (or just fail). I have seen power supply parts oscillating at high temperature because the stress on the die caused the internal reference to oscillate
I posted a question on this forum back in December last year about the effect of lifetime on components, saying it should run into thousands of years... and got hounded for my answer. I have looked into it since and spoken to many packaging guys, chip designers and QA people and indeed it is thousands of years as far as the silicon is concerned. Mechanical failures is another matter though