The new Ivy Bridge chipsets have all sorts of thermal management goodness built in, which should help with any heat issues. The new processors are substantially more efficient as well. Yanking the DVD should also make it easier to provide the internals with cooling, even if the case gets thinner. Newer monitors tend to run cooler as well, as do newer hard drives (especially if they go with SSDs as standard equipment for the boot drive, which Apple just might).
9/7/12 at 3:03pm