My 2 Cents:  If you are are experiencing any similar issues you might want to go and have your machine checked out (who know, you might be one of the lucky ones that gets yours swapped out for a new one)….but in my mind it’s expected that a machine that thin with a desktop CPU inside would run a little hot…but it is nice ;O)

We’re still hearing reports of MacBook Air overheating woe, well after an EFI update in April that was meant to address some of those problems. While hot computers are nothing new, the MacBook Air starts shutting down cores and offloading processes when things get bad, which means users are faced with aggravating stop-start freezing until they can manage to cool the computer off — or just put it to sleep and let it “rest.” Apparently some people have traced this back to the age-old misapplied thermal grease problem, but that’s hardly a solace for the average consumer trying to convince Apple to fix this thing for them. We took a MacBook Air that was acting up into the Genius Bar and Apple claimed it couldn’t reproduce the problem, though we have heard cases of Apple replacing the computer for users. We’d be curious to know just how many Air users are having trouble, and if the X300 is experiencing anything similar, so let us know in the comments.

MacBook Air users still faced with overheating problems? – Engadget