The newest iPods (iPod Mini and 40gb at least) offer this ability with audiobooks that you buy from Audible.com. The speed of playback of audiobooks from that company can be adjusted faster or slower.
I had a Creative Nomad Zen with 30GB of storage and it had the ability to adjust the playback speed of all tracks while not affecting the pitch of the sound. But doing so does reduce the battery life.
I hope that helps.
Looking for an mp3 player which can be sped up in order to listen to audiobooks. Somthing akin to the tape players used for Books on Tape for the blind. we can listen a lot faster than someone reading aloud.

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