Being developed by the same people who did SingStar, their are only a few details released thus far, as well as a couple screenshots courtesy of x360a.
-40 songs on the disc, with DLC tracks planned
-Record your own songs in a "Jam Session" (8 separate tracks)
-Online co-op, versus, and battle of the bands modes
-Large drum set with six pads, plus pedal
All released screenshots can be viewed here. For the drum set, feast your eyes:
With Rock Band, the Guitar Hero 4 announcement, and now this, your 360 is going to span an entire room to contain all these drumsets and instruments. I think this is a little too much. Perhaps I will just wait until Harmonix releases a patch to include a Record Your Own Song option.