Above, a pic that supposedly proves that, via the wonders of DLC, you will indeed be able to play as both Vader and Yoda in Soul Calibur IV, regardless of which version of the game you buy. It's certainly far from conclusive proof, in that all it proves is that Yoda is on the left of a digital image while Vader's on the right, but far crazier, far less likely rumours have turned out to be true than this one.
Update - Perhaps sensing the internet's trepidation, the source of the pics - GamersReports - have thrown a video clip up on YouTube, showing Yoda and Vader going at it. You can see it after the jump. Still not proof, but it's fun to watch regardless.
Vader Coming To 360 Soul Calibur IV As Download [GamersReports]