I have the same problem. I am playing FNV for 90 hours now, I'm at Level 30 and I'm stuck in the
Hoover Dam!! The
two doors in the Visitor Center which leads to the outside Hoover Dam cannot be opened (you
need to have a key, which is not the one Lt. Moore is carrying) and the roof access is just non-existent. I have no idea what is the problem, but I'm sure it's not occurring everytime you do the Eureka! quest (because
it's the second time I've tried it, but the first time I was not strong enough so I've chosen to continue leveling up in the Mojave Wasteland until I was level 30.)
It's a shame to have such a bug in a such important quest (
you can't even finish the game!!).
Also,
I've tried to reload an old save, but the doors were already "locked". Seems like it was like that for a long time now.
Hoping developers are reading this, because I'm pretty sure this is one of the biggest (I mean, the most important... you cannot finish the game, damnit!) bug you may encounter in FNV. I'm not angry at Obsidian Entertainment or Bethesda Softworks, I'm just kind of disappointed of such a lack of "polish", if I could say. It wouldn't have killed anybody to release the game a month after to fix most of the bugs (the game would have received better reviews, too...).