Thanks for starting this thread, Jes.  My new flash player just arrived yesterday and it holds up to 8 hours of songs.  I bought it so that I'd have more variety at the gym.  These posts are giving me ideas.  
BTW, some of my favorite songs for a 4mph walk on the treadmill are Brown Sugar (by the Rolling Stones), Disco Inferno from SNFever and Believe (by Cher).