I've got one of the new slim PS3s. I'm not getting much of a drop in framerate in the overworld (outdoor sections). However, I am having framerate problems in some dungeons. For example, the framerate is pretty choppy in Snapleg Cave.
I haven't yet found that things have got worse with play time (I'm up to 12 hours now) or game save size (my save file is just under 5mb).
Having spoken to friends who have the Xbox version, they aren't experiencing the same framerate issues. This leads me to conclude that the problems we are experiencing are just based around the X360 version being the lead version and that the PS3 port isn't as good as for some other titles. For example, I have Battlefield 3 on PS3 and it runs great.
One other thing I would draw everyone's attention to is that Eurogamer.net will be publishing their PS3 vs 360 vs PC Skyrim comparison today. They've done this for loads of other games and it's a really good quality bit of research into the differences including framerate measurements. I expect that they'll show that the PS3 performance in Skyrim is not as good as the 360.
P.

