I generally make a rule for myself not to use my IPOD when riding so I can hear cars but if it is safe, and I am really bored I might for a while. Mostly, the IPOD is my luxury item for camp when I'm done for the day and relaxing in my tent.
I have an eclectic mix depending on my mood but enjoy classical suites from Chopin, Tchaikovsky, Mozart... as well as Leonard Cohen, Ella Fitzgerald, Chris Cornell, Pearl Jam, REM or other good vocalists.
Dark Side is my number one default for descents while mtbing.