For the record I'm going to save right in front of the bunker, go delete Dead Money, load the game in front of the bunker sans-Dead Money... save again... then re-download Dead Money (whew) and see if that works. I'll come back later and say whether it fixed it or not... I know I'm not the only one with this problem.
No, you're very far from being the only person with the issue. It's common, the fix stated by 404s works. </topic>
EDIT: And no, clearing the cache will not remove your title update for New Vegas. Take a look on your hard drive and you'll see the update is quite visible. Other title updates are hidden files. This one isn't.