I've been kicking around some ideas for getting 3D effects for 2D games, without actually rendering objects in OpenGL.
One idea is running bumpmapping in the postprocessor. Render image once with contour maps and use that as a bumpmap, then a second time in the normal color, and let the postprocessor run a pass on it, with the end result of a bumpmapped image.
This could be very cool for such such things like dungeon floors in Gauntlet-like games, and would very likely give a gorgeous look for sprites, which would react to light in ways ordinarily impossible for 2D sprites.
Because of the restricted problem domain it might even be possible to do something similar to raytracing, again on the shader unit.
Any thoughts?