I'm expecting the worldwide lead to swap mid to late 09. The main reason I'm guessing a little earlier is Blu-Ray. I think now the format war is over and HD TV is around in increasing numbers of channels, people are going to be more willing to buy into the whole high def thing. And there aren't really many reasons to buy a Blu-Ray player that's not a PS3 right now.
Additionally, I think in traditional Playstation style, we're going to see much improved graphics in future generations of PS3 games. Developers have long been complaining about the difficulty of programming PS3 vs the ease of the 360. Another way of looking at this is that current games are already getting quite a lot from the 360 as it's just like programming for a PC, while they're probably nowhere near 100% yet on PS3. The same was true on PS2, and games coming out even now are still making noticeable improvements on what has gone before.
I think this Christmas the sales on Blu-Ray are going to start picking up to meaningful amounts and that will start eating into the 360s lead pretty quickly. The PS3 exclusives I believe are going to look better than games available on the 360, although I expect the big budget games to remain primarily cross-platform.
