Default resolution next gen is a big question mark due to several things:
1) We don't really know how powerful will the next gen be. A tenfold increase in performance will probably mean that the amount of sub-HD games will be reduced compared with this gen.
2) 3D penetration. 3D has a performance overhead compared with 2D, so games that aim to fully utilate 3D usually end up with higher resolution 2D modes (for example, Super Stardust HD or Motorstorm: Apocalypse, running at 1280x1080).
3) Performance differences between major consoles. If one console gets significantly ahead of the others, while not becoming the market leader, we could end up with a situation where games are designed for weaker consoles and then get up-ported to the stronger one via resolution increase, as it doesn't require much work and provides relatively good results. Similarly, we could see down-porting, where games on weaker consoles end up with lower resolution (whether that's sub-HD or just HD depends on factors mentioned above).
The one big advantage that this gen has brought us is the creation of new antialiasing methods (MLAA, FXAA etc.), that significantly decrease the amount of bandwith required to perform anti aliasing, allowing for higher resolution instead.
Realistically, with a tenfold increase in performance we can expect that games move towards to 720p and above. Even today, sub-HD games aren't exactly a majority. They may indeed appear next gen, but in slower numbers and I bet that the number of games with resolution greater than 720p will significantly increase.