I did a quick search of the board to see if there was a soultion/explaination for this, but couldn't find anything. So:

My rep with the ncr good, I've killed everyone at bunker hill, and the ncr is thanking me wherever I go. When I approach the dam for the quest "you'll know when it happens" I get shot at by a legionare assassin standing on the road next to the visitors centre. This happened after I fast traveled, so thinking this was a fast travel related bug, reloaded a previous save, and walked in. On the way in I pass friendly ncr soldiers, BUT as soon as the assassin shoots at me the ncr soldiers turn on me, the quest fails and I become villified by the ncr. I can't see how it is possible at all to complete this quest at all if it instantly fails when I come near the dam. (also, no I am not wearing faction armor, no I don't have a companion, no I didn't shoot the assassin first)

Any ideas are welcome

I..I got nothing.That's totally new. Maybe..uhhm,companion related? Drop minions off somewhere and try again?They tend to create hostility in factions for some reason. Or if you have many different saves take one from a while back,get the quest and try it then,before you did a bunch of other stuff. Maybe you pissed off the wrong monkey somewhere? I dunno.That's a doozy.

Or,your game is glitched to a point making it useless.It's possible,new game? Been there,trust me,not a happy camper. You can thank Bethesda/Obsidian for that.

oh thats simple you just.....wait what?
why is there a legionnaire even there on the side of the road?
i just did this quest today....there was no such person anywhere near the dam
i even fire my gun into the air while running around and nothing happen negatively
this is very confusing
is it possible you have an invisible companion? like did boone disapear at some point in your travels? and he is technically with you but not?

the only advice i can give would be

1. double check make sure no companions are with you(try getting someone to follow you and see what they say if they say "you seem to have 1 too many" or something like that you have a bugged companion
or
2. go sleep for a week in a bed somewhere then try again

I'm sure 100% I don't have any companions, they have a tendency to die on me (hardcoe mode) not long after joining me. I gave that quest a break for a while to kill things for the thorn, some random side quests, and when I came back it was exactly the same. thinking back, that assassin has always been on the dam every time I went there, just now though is he causing a problem. I figured a work around, used a stealth boy to travel into the visitors centre. It worked to start the mission, but then he was just standing on the far side of the tower the ncr sniper is on. I was able to kill him quickly with a ballistic fist without the ncr turning on me, but then as the vertibird flew in the game crashed, again (6th time in one day). I've given up on this game until they fix it.

*SPOILERS*

thats really odd...
when i did this i entered no problem (the assassin was not there)
i went into the visitor center talked to the CO, started the quests asked for full access
logged onto the computer in the same room got the info
talked to the chick up stairs with the glasses she tells you her engineer friend in missing(i didnt bother finding him)
went outside talked to the CO again the vertibird flew in
president kimballs vetibird flew in, landed safely
i began watching the 3 marked locations from the top of the stairs
president kimball starts his speech
you will eventually see an NCR sniper get thrown from the roof of the sniper location behind president kimball
go investigate the body then climb the ladder
he will try to talk his way out of it then say "screw it your to much trouble" and become hostile
then use the radio at the top of this tower to call in the security breach (haul ass back to the stage you will see the engineer i referenced earlier attack president kimball)
his black armor NCR will take out the engineer no problem
again haul ass up to the presidents vertibird examine the vertibird and remove the bomb(i dont remember if its repair or science skill you need)