The song used for the trailer of Watchmen (which I still haven't seen yet!) is "The Beginning Is The End Is The Beginning" by the Smashing Pumpkins.
It also doesn't seem to be on the soundtrack, which is a bit disappointing.
It's pretty easy to find the song on youtube if you want to take a listen. I won't link to any specific copies, since songs don't always last that long on youtube. Here's a link to the search, to save you some time and all that The Beginning Is The End Is The Beginning
This song is also VERY similar (;-)) to the Smashing Pumpkins song "The End Is The Beginning Is The End" that can be found on the soundtrack for the 1997 movie Batman & Robin.