That Japanese site linked earlier has tons of games covered with direct feed screen comparisons, and it's a very good resource for someone looking for differences.
If you go here:
http://d.hatena.ne.jp/yoda-dip-jp/searchdiary?word=*[Game%20Compare]
and keep clicking the < arrow pointing to the left link, located on top of each update's title and date, you can get to earlier pages.
By going through it quickly, there's plenty of examples where PS3 version looks flat out softer for various reasons, but I also saw several examples where the image quality looked nigh identical, like on Bionic Commando, Fifa 09 (PS3 ver actually seems to have some extra post processing on this one) SC4 (PS3 screens actually look a bit sharper here, but it could be due to less AA or just different AA method, not sure), or Burnout (again very slightly different, X360 has some more dithering going on)