Understand, I have no experience with such things... but Sennheiser makes "active noise compensation" headphones. I believe that they sample noise from your environment, invert the waveform, and send the inverted signal to the speakers, thus canceling out most sounds. You can find a review here.

For my own part, I truly sympathize. A certain amount of background noise I can ignore easily, but sometimes conversations or certain other sounds distract me. I especially remember one time when a deadline was approaching, the non-technical folks began celebrating while the hackers were crunching. I just about bit a fairly nice fellow's head off.

These days, if I can't get silence, I generally listen to Bach; it's nice to have my disks onhand. I can code to his harpsichord concerti very well, occasionally pausing just to listen. Some Mozart's okay, but a lot of his stuff is a bit too emotional.