Good question. I live in Maryland, but it never stays too hot there. Not hot in the level that it'll make a big difference in electronics.
On the other hand, the most I've seen my MBA 13" Ultimate work was when showing 1080P flash video off of Youtube with flash 10.1 (Because the CPU was pretty much pushing the whole thing). 10.2 beta actually uses the system's resources much better and hardware accelerates the flash video so that your system stays pretty cool, even with the most demanding 1080p flash vids. You might hear the fan kick on for just a bit with 10.2 beta, but in 10.1 the fans and the heat seem to be at the highest. Still seemed to truck along, though. So far, nothing else demanded more to heat up my Air, and even when it was (with 10.1) it never got hot to the touch. Just a bit on the high warm side.
I'm not using a skin on it, though, so I don't know if it'll make the difference. I've tried games on it, and it seems to not cause as much work and heat as the 1080p flash video did. The fans will activate, but they seem to run at a lower speed with games. Probably because they're more on the gfx card and it would seem the CPU is where the heat really is.
The skin looks nice. It's also very snazzy. I like the fact that you can still take it off, in the future. I wouldn't want anything permanent. Just something that can leave the unit like brand new after the skin is removed.