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. (http://hackaday.com/2011/02/27/layering-pinball-audio-using-parallel-wav-shields/) 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.