2D here.
This is because my DREAM is currently to have a clone of a game that really hasn't been redone nicely since release and death in 97/98.
People who have the time and team can possibly do their dream in 3D, but if it can be done in 2D, it gives that extra time to work on things that may matter more, like game-play, bug fixes, cross-platform, etc.
With many nice 3D engines popping up for XNA 2.0, it shouldn't be too long before they will be similar to building (time-wise).
For those without 3D math skills that might need it, there are always great sites out there listed on google that can give equations to just plug into the game, and go. Ie stuff for physics, lighting, waves, etc.
3D doesn't always have to be setting the bar high either. People could be using 3D part of XNA to make a sidescroller, just with cool effects. Game on PSP does this...can't remember which.