Toss it in a Ziplock bag in your center jersey pocket. My Blackberry lives back there on a semi-regular basis. Plays my music (through one earbud for safety), takes my phone calls if I get one (rarely do), and tracks my route via GPS while riding for download and analysis later. Ziplock keeps it from getting coated in sweat, and the jersey pocket keeps it close enough I don't need bluetooth for the headset, and it doesn't have the chance of "rattling off".

Works like a champ for me. My buddy has recently adopted the same "system" for his iPhone with equally good results.