You've been able to sing or hum a tune into midomi.com and have it take a shot at guessing what you're crooning for a while, but today this novel tech that uses a user-generated database of recordings for comparison has found what I think might be its perfect home on the iPhone. Instead of lobbing up "Happy Birthday" or any similar, piece-of-cake pap, we thought we'd give it a more intermediate-level test suitable for our times.
Yep, aside from a few server hangs, the thing works as advertised.
UPDATE: As many commenters pointed out, Shazam is a similar app. Ran it through the Rickroll humming test and it couldn't come up with a match, but it does seem like it's highly superior for picking up and ID'ing ambient music, like the Al Green playing on the office soundsystem right now.