Gamespot has done a second annual comparison of games on both the 360 and PS3. They took screencaps of the same points in both versions of the games, in as similar conditions as they could get (naturally this was almost impossible with GTA IV).
What does this prove? That developers know how to get good graphics out of the system. It does not prove which is more powerful*, as the list of games were designed mostly with both systems in mind.
As for which is better, it depends on the games. A couple have a gray filter on the PS3 version (Assassin's Creed, Rainbow Six Vegas 2). One has a gray filter on the 360 version (Orange Box). A few just have the shadows moved around (Need for Speed Pro Street, Call of Duty 4, Burnout Paradise). And one has almost negligible differences (Madden 08, although there is the framerate issue).
Then there is GTA IV. Yes these are screencaps. but the PS3 version seems to have a green filter in a couple of them, and has an orange filter in the others. Also, the cars look a bit rougher in the PS3 version, although I don't know the terms to explain it.
So what would it mean to someone who hasn't gotten either system yet, like me? Well I'll just speak for myself. It means little. I'm not a graphics whore. To me, how much fun I have is more important. I just thought it would be interesting to look at the games.
Conclusion: Both systems have good graphics, but of course the fanboys won't stand for such a compromise. Gamespot compares dual platform games.
* And even then, it won't shut the fanboys up.