Endagadet and Homemediachoice recently teamed up to give reviews of the PS3slim.
While i'm not going to post the entire thing, I think that the heat issues are best explained by geeks that are better than me:
"In operation
When you power up the console, the first thing you need to look for is a system update. My console found v2.80 and within a few minutes this had downloaded and installed. V.3, which brings some extra functionality, has yet to make an appearance at the time of the writing.
The original PS3 was significantly quieter than its Microsoft rival when it came to operational noise. While thte Xbox 360 sounds like a hyperactive vacuum cleaner, the PS3 maintained a dignified hum.
This model, at least initiall, is quieter still. However, heat builds up quickly during use and the fan noise accelerates accordingly. Midway through X0Men Origins: Wolverine, I noted that the console was disgorging a lot of heat through its rear vents, and at that point, in terms of fan noise it did not seem to be an improvement on what's gone before. The huge 95mm, 17-blade fan beneath the hood clarly has its work cut out...."
