I'm on my second set of Sony MDR-NC5 that hsilbiger also recommended (my first set was ripped off in Milan
http://www.flyertalk.com/forum/frown.gif).
You can find them for about $50-$60 on eCost.com or on eBay as others have said.
I think that they are almost just as good as the much more expensive Bose, so the minute difference in quality does not justify the price difference.
The NC headphones block out all the low level rumble that make you TIRED on any flight, short or long. Several years ago when I first tried NC headphones on UA C-class to Europe, I went out and bought my first pair of Sony MDR-NC5 immediately, and I have considered them to be one of the best investments that I have ever made (just to allow me to arrive at my destination a bit less tired).
Back to your original question, I know that UA, BA, LH, SQ, all offer NC headsets, but on C or F only (ouch), so you'll need to buy your own NC headphones if you are stuck in Y.