Well for one you're stating the obvious. The design of the iMac does have its limitations when compared to a tower with all sorts of space for airflow inside of it, and space for really big heat spreaders. The dark spots seems more of an isolated problem, it could have been caused by some computers being built less effectively than others-- bad application of thermal paste could be one reason. I've had my i7 iMac for months now, and have underwent hours upon hours of intensive video encoding that maxes out the cpu the whole time. The temp of my i7 never got above 69c before fans started kicking in, and it only took the cpu fan going from 940rpm to 1300 rpm for the cpu to be cooled down to under 60c again. The i7 also idles at 39c on my iMac. I've played demanding games like dirt2 and starcraft2 on the iMac, the gpu has gone up to 75c, but never higher. You can even rev the optical drive fan up to 2-3k rpm and the gpu will never go above 60c.
So I for one don't appreciate acting as if the new iMacs are ticking time bombs, because they're designed well for what Apple chose to limit them as. They run warmer to other computers because of their slim case and really low fan rpms-- Apple choses quiet computers over ultra cool computers. That doesn't mean they're bad on the cooling department though. Keeping internal parts below 30c all the time is fine and dandy, but doesn't really improve lifetime expectancy. Temps of 80c or below for gpu and 70c or below for cpu are perfectly within normal operating temperatures, and you could force them to stay well below that if you forced the fan rpms up to speeds you'd normally see on PC rigs. The new iMacs have pretty good cooling performance, no reason to declare jihad against Apple for the occasional lemons they produce, such as the ones in this thread that are experiencing burn marks on their screens.