I've been wondering about this for a while. My first real MP3 player was a cowon U2, and the best part about it was the play speed. In the EQ section where BBE and all that jazz were there was a play speed that went from -10 to +10. I have no clue what the settings and speed for each one were but it was fun as heck to listen to songs in different pitches and speed. I think some versions of rockbox have this feature depending on the hardware, but I don't think they adjusted pitch, just the actual playback speed. I might be thinking of some default firmware though.
Anyway, here are my questions.
1) Is speed/pitch adjustment somehow hardware based or is it simply software?
2) If it's software, does it put a large hamper on CPU usage?
3) And if it's not a bad idea, how hard would it be to implement and how would one go about doing it? I don't have much (any) programming experience but I'd be willing to go dig around and learn some stuff to get this on my X5L.