Allright, I found a possible source for the damage done to Vader. It's in a new Fanity Fair article found here:
http://www.vanityfair.com/culture/features/2008/03/lucas200803)
Look at page 3, last bit of the page. It describes what happens. But in short:
Before I leave, Blackman and Stafford take me to a workstation where Adam Piper, a senior-level designer for The Force Unleashed, is playing through the prologue on a PlayStation 3 console. The prologue allows the gamer to play Darth Vader, who has gone to Kashyyyk, the Wookiee home planet, to hunt down a recalcitrant Jedi.
The article further describes Vader attacking a certain 'walking carpet' race. Knowing those guys are insanely strong, they might be the source for the damage :)
Playing Vader....this is looking more awesome with the minute! :D
PS: Might explain the rumors about the 'Vader game' as well...