No, I haven't heard about the slim design but it wouldn't work.
I wasn't so sure of it at first, but I have got used to the fat design of the PS3. The fat PS2 was ugly and had too many straight edges, this forced them to create the slimmer one. The PS3's design does start to grow on you after a while, the PS2 was just plain disgusting.
The only time my PS3 gets hot is when I am playing a game. I left my PS3 on for about 10 hours (longest I have left it) with F@H running, and I came back and it felt like it was turned off. If I played a game running for 10 hours, I would barely be able to touch the console. I remember at Easter I got a bar of chocolate (like you do) and put it in my drawer. I came to eat it and it had started to melt. Why? Because that drawer is directly under the PS3.