Trust me, you wont be using an MP3 player at all. The only thing youll be able to have with you is your wallet and an address book. Everything else will be put into personal effects and not be given back to you the last week youre there.
You wont even get to use one in MCT/SOI, since youll always be in the field.
Trust me, Im a corporal in iraq.
EDIT: ive got an 80gb black ipod video sitting next to me. so far its survived 7 months of iraq in a small rubber case. Make sure you get something that is compatible with 220v electricity, im sure most mp3 players are. Youll be over here in a year or so depending on your MOS and what unit you check into.