Anecdotally, my experience jives with what Technut said.

Totally cool body with 2007. With 2008, sometimes would get warm (almost hot) under the battery bulge of the n800. Never hot, like some mac laptops I know, but warm. Downgraded back to 2007 and it never got hot again.

I haven't grabbed the 2008 beta, so I can't say whether I would have the same issue. But to repeat, it was warm (like would be nice in your hands outside on a cold day) never actually hot.

With winter coming on, maybe this is a 'feature'