Nintendo has released a schedule of upcoming Mega Man 10 (WiiWare) DLC, and surprise, surprise, it confirms Bass as the third playable character, among other features. The details below:
I imagine we'll be learning more about Bass's specific abilities in the near future, but if rumors hold true, he'll have the same abilities as in MM&B: multi-angled shots, dashing and double jump.
Thanks SantanaClaus89 for the tip!
UPDATE: Reading over MM10's Operations Manual, one can find not only sprites of Bass, but Treble as well!
Also, here's some additional details over Bass Mode according to the Operation's Manual: "Bass can shoot in 7 directions. Some of his abilities differ from Mega Man's. Not compatible with Rankings or Challenges."
More info as it comes our way.