Buy a PS2.
I second that idea. For the amount you would drop on a 3 and a game, you could get a two, a couple of memory cards, multiple controllers, and about 15 really, really good, deep, and absorbing games. I am still buying PS2 games, playing them, and killing lots of time on them. I just picked up Persona 3 Fes, and even though I've already played through the original copy, I'm still loving this game. Persona 4 has been announced for PS2 as well. I'm sure you could ask anybody for suggestions on good PS2 games and we'd all give you a laundry list that would probably last you ten years.
I've never been an early adopter of systems, and I would say that if a system is still pushing $500 then you would be an early adopter for buying it. Hell, I finally went out on a limb and got a 360, and as we speak the face plate and HD are on my shelf, the unit is being serviced. I don't think the PS3 will have such a problem, but still, it's a big investment of mine that isn't even here.
And like everyone else said, MGS4 won't stay exclusive. And I doubt FFXIII will either. I am really excited for both games as I have been a Final Fantasy fan since the NES days (I bought the PS1 just for FFVII). I am not worried about the fact that both games are 'exclusives' because I firmly believe both will migrate to the 360. If they don't, well, it will be far enough down the road that I will be able to afford to buy them along with a PS3.