I think Microsoft is in a really good position right now and how much they decide to invest in gaming in the future is probably going to depend partially on the success of Windows 7 and how well they manage to contain Google out of the PC.
The better those strategic areas of their business do, the more money they will be ready to throw at the Xbox and at games development again (and the less well those do, the less willing they will be to throw cash at the Xbox).
Nintendo is in a good position too but he's facing another challenge.
The main issue they have in my view is two folds :
- They have to watch rising competition from the iPhone for their handhelds business.( but their strong franchises should allow them to retain a strong market position)
- They have to be carefull about not moving their console base to their handheld base. ( it's happened in Japan, and you could make a case it's starting to happen outside of Japan these last few months( DS sales up, Wii sales down)).
Ideally they want their users to have both console and handhelds, not handheld only and the fact that they are slowly making everyone of their franchise available on handhelds is making some users only purchase handhelds.
Handhelds have the others advantages that they are a lot more friendly for casual users. ( one of the issue with consoles for the new casual gamers they are bringing is console need a tv ( which means the rest of the family can't watch it if you use the console), handhelds don't have that issue and are far more convenient for casuals.
I think Sony can manage to salvage the PS brand for the console market if they don't make the same mistakes next gen ( pricing mainly).
I do think the PSP is facing a huge risk of disappearing in the next years.
Most of the non gaming capabilities of the PSP are available on the iPhone which is a lot more popular, piracy is killing the gaming software on the PSP and Sony really doesn't have strong franchises on the PSP...