SPOILERS!!!
This is a mission you get during the "For the Republic" mission line for the NCR, while president Kimble visits Hoover Dam. While there may be a couple of ways of completing this, here is the method I used.
Once you are talking to Ranger Grant, ask all question you can of him, make sure you get his itinerary. Then, access the computer terminal next to him(requires science 50), to determine some one has hacked into the system. Now talk to ranger Grant initiate the president event, and while its flying in head to the ladder behind Ranger Grant that leads to the top of The visitor center, where the copter will land. Once it has landed, an engineer will begin fiddling with the plane. examine the plane, and with an explosives or repair of 65 or higher, you will find a bomb. Quickly go tell Ranger Grant, and the speech will be ended(thank god, it is the most boring speech in all of the post apocalypse), Kimble will be hustled to his copter, and the achievement will pop.
Note that there are a lot of "waypoints" on this mission, I imagine there are other methods to complete this, such as possibly catching the sniper, but you would probably still need to remove the bomb anyways, so this is probably the shortest way.
Edit: This can also be done during the Yes Man storyline if the NCR are friendly towards you. Thanks Radiant Viper for the hint!
Edit... This being a Fallout game, there are a number of ways of completing this mission. So I have posted some alternate solutions that have been given by people in the comments section...
Edit:(taken directly from comment, thanks VoodooAK !!)
You can also go to the closest tower behind the stage area that President Kimble will speak at. From the ground you can see a NCR Ranger posted there. Go up to the top and join the Ranger, after a bit into Kimble's speech another NCR Ranger will come up through the hatch and will be shown as an enemy (he attacked me on sight). Kill the enemy Ranger (presumably spy for the Legion), then return to Grant and tell him about the sniper. He will end Kimble's speech, but first a person will run up and try to kill Kimble but is stopped by the Rangers. The 2nd assassin actually killed a Ranger when I played.
Edit(11/17/10): Another solution from BearsinGames...
I went into the storage closet under the stairs up to the roof, and there was a pool of blood. This required a Perception above 6 to analyze, and gives you the result that someone in an Engineer suit is going to attack the President. During the middle of his speech, a third engineer joins the crowd. If you talk to him and insist he comes with you, he attacks you. After killing him, the President escapes safely with no further interactions needed.