Soundhound is the fastest music recognition iPhone application I have ever seen. I like Shazam but this kicks its butt big time. Sometimes it will identify a song within a few seconds, instead of listening for a preset time and then sending it in for verification.
The latest version lets you bounce back and forth between Soundhound and iTunes for faster shopping of songs too! They also have a free version here if you want to try it out first, but it's worth spending the money on the full version.
Soundhound is easy to you use, and I've yet to find a song it doesn't know!