The phat PS3 is a very sexy looking device. But with that sexy-ness comes the hassle of maintaining it. As many ps3 owners would know, this thing attracts dust even if its in a relatively clean area. The plastic case literally gets magnetized enough to draw dust to it. Also, if you look around the case it has plenty of little holes and divets where dust can get trapped. When ever i vacuum my room i usually would take the nozzle and try to suck up the dust off my ps3 as well as well as my 360. 
 
Now after see screens off the newer model (Sony isn't really calling it the SLIM just yet), and see kotaku's "Five Senses Test," the new PS3 looks to have a more cleaner design with vents only in the back. The plastic also looks like it wouldn't attract much dust either.
 
Call me crazy, or a neat freak, but thats really the only reason i would trade in my phat PS3 for the newer one. I still love it either way.