When Google Maps 3.2 was released a bit ago I got all excited about being able to overlay the NYC subway on top of the map view. I was saddened to learn that this feature was not available at launch though it was available in other markets.
I just checked yesterday and it looks like Google has added it! Just fire up Google Maps and go to Menu->Layers->More Layers->Transit Lines. You will then see the NYC subway map overlayed on the regular map like you see in this screenshot.
If you don’t already have Google Maps pick it up at m.google.com/maps