Well I wouldn't exactly say that was a defining reason to buy a PS2 seeing as there are other (certainly better) games on the way for Sony's machine. Not to mention the fact that in this volatile games industry of ours, exclusives rarely stay exclusive for long.
I think i'll give San Andreas six months, tops, before you see it on 'another' system.
I agree totally with WDC in that Killzone is looking amazing, with the exception of some nasty framerate issues which will hopefully be sorted out in time for release. To be honest, it's not actually Killzone i'm looking forward to from that developer as they are also producing a title called Shellshocked: NAM 67' which is a super-visceral, hyper detailed depiction of the nam war which looks simply stunning and could shape up to be a sleeper hit this year.