If DMC4 does reach the xbox 360, more people can play the series which is always a plus. Concerning exclusivity, I think Sony really needs to step up and show game developers why they should work exclusively for the PS3. Otherwise, they really don't have much to offer...if all games go multiplatform (between xbox and playstation) then there really isn't a point in buying a console that costs $200 more just for the same gameplay and graphics.
However, I doubt Sony is "doomed" or "dead"...they are far from that. PS2 launched pretty badly as well however they ended up being the best last generation. I think most people need to give them time...2007 is supposed to be a pretty profitable year for them as long as the big name games come out and get good reviews. Anyways, that's my opinion...who knows what could happen...this generation in particular is extremely unpredictable 
yo 
