Dance Central's Twitter page just said new DLC is
coming Dec. 21 so Harmonix may have posted it early on
accident. I've seen it happen with Rock Band as well.
Where did you hear about it?
EDIT* Yep, apparently the game did post it. From DC's
forums, "I was so surprised to fire up the game this
morning and have it tell me that new DLC is available
from Janet Jackson, Blur, and Rihanna."