Has anyone built a stereo version of the waveshield?
Not that I'm aware of but there are people controlling MP3 players with Arduino. - http://arduino.cc/forum/index.php/topic,49979.0.html
There is this. Not strictly stereo but interesting nonetheless.
Boards such as the rMP3 can play stereo MP3 files up to 320Kbps.
As the waveshield is only capable of playing 22KHz samples, I suspect that no one can make a stereo version unless you want to drop down to 11KHz samples.