Since faction armor isn't involved it does sound like a glitch, but out of curiosity what's your reputation with the Legion? Perhaps a high reputation with the Legion can make the NCR hostile even if your NCR reputation doesn't reflect that.
My Legion reputation was Neutral throughout the entire game to that point as I had ignored them. It did not appear to have anything to do with that.
I should note that I was able to complete the NCR quests today by killing the NCR troops that were attacking me inside the Hoover Dam. Once I did that, I was able to put on the Salvaged Power Armor and make it to the Colonel. After that, I have yet to be shot. This makes no sense, since I shouldn't be able to stop the NCR from attacking me by killing them (previously, wearing NCR armor from an NCR soldier I did *not* kill had no effect). In any case I can rather freely remove the armor now (I can't interact with the BoS wearing NCR armor) and the NCR now recognizes me for saving Kimball.
I still believe there is an issue with NCR hostility. Also, for another NCR quest (Return to Sender) I was unable to communicate with *any* NCR personnel *anywhere*. At Forlorn Hope, everybody simply acted like a hunting dog that had spotted somebody - they froze, mouths hanging open, and would not respond to me. They would not even respond to me attacking them, and only by killing one and attacking another was I able to break them out of their frozen state. Some traveling let me clear that hostility. I mean at this point it seems like you solve problems with NCR NPCs by killing them.