There's a bit sent to the competition port that tells it it's in autonomous mode. PB_Mode is the bit, and it's aliased as auton_mode, user_display_mode, and comp_mode. there's a lot more in the manual about it (even some example code to make it easy to switch from autonomous mode to user/competition mode...)
if you need ANY help feel free to e-mail me or PM me or IM me...take your pick.
*jeremy