Thats right, one of our readers was able to get a hold of a few moments with the author of the Space Marine codex to ask the one question that has been digging at us for week's now. This answer is good enough for me.
Do Grav Weapons cause an extra hull point when they hit an already immobilized vehicle?
via Chris, and of course thanks Robin Cruddace for answering this burning question.
I’ve just got back from games day UK, where I spoke to Robin Cruddace about the question you raised over grav weapons immobilising a vehicle more than once. He confirmed that if a vehicle is immobilised more than once by a grav weapon rolling a 6, then it *will* loose an additional hull point for being immobilised while already immobilised, just as if you were rolling normally on the vehicle damage table. I asked him to see if he can get this in the official FAQ, so hopefully we’ll see that appear sometime soon.