I'm having some problems with the PS3 too. I can't finish MGS4 because it won't read the disc. I can play for a while, well actually I can play for as long as I want to, but if I enter the menu and go to the item screen and then try to go back into the game, that disc loading thing appears in the top left corner, and it never goes back into the game again. All I can do is reset the game, but when the console starts up it won't read the disc, no matter how many times I try, or even if I switch game. It can read DVDs however, which is kind of fucked.
I've been able to work around this problem earlier in the game, but now I'm at a point in the game where, if I try to progress further, it switches to a cut-scene and the loading thing happens again, and I'm fucked.
Apparently this is a pretty common problem with the 40GB version of the PS3, it's a fucking shame with such a ridiculously expensive console.
Oh, also, when it stops reading discs, I can wait for around 2-3 hours, and suddenly it starts reading discs again. How fucked is that?