What I did was backup my Skyrim save data file and the the character save file, completely reformat my PS3 HDD, then move nothing back onto it except the Skyrim files.
I can report that the lag issue has disappeared by about 99%. Also my load times are about half, only 1-3 seconds to go in and out of any building in town (tested in Windhelm).
I'm sure that upgrading your PS3 to have an SSD would be a nice (but costly) addition, but perhaps try to reformat your current HDD first if you're like me and only want to spend money as a last resort.
Hopefully it works for you all as well as it worked for me.
Obviously it will get slow/laggy again if you clog up your newly formatted HDD with a bunch of stuff right away so the key is keeping your HDD clean.
Cheers,
-reicher_89