I think the next generation of consoles will come out when widespread 3D TV adoption allows for 3D gaming. The PS3 and 360 were developed to take advantage of HDTVs, so unless there is something driving a new generation of consoles I don't see the business case for developing one. Graphics are, for the most part, good enough on the PS3 and 360, so a whole new generation at the same resolutions doesn't make much sense (although a move to full 1080p support would be nice).