So, I'm at Camp Mcrarran (SP?), and I approach a door on the upper level with 2 troopers next to it. The door opens up to the Mojave wasteland. Anyways, I approach the door, and all of a sudden every trooper in the Airport except Hsu starts trying to kill me. Needless to say, they were successful. I am in good standing with the NCR, why would they want me dead?

I think the same thing happened to me.. I remember there being 2 NCR standing by a door in a ready position and when I went up there they attacked me. I can't remember exactly where I was though.

Yea.... maybe don't go near the door?

I'm going to take a wild guess and say you're trying to board the NCR monorail to the strip. It's NCR only. Put on an NCR disguise and you can board.