Few quick points: make ABSOLUTELY, POSITIVELY you SIGN for a guaranteed aviation slot. The OSO might say you can compete for it at the Basic School. Those slots are few and far between and the decision on who gets them has little to do with aviation.
(Years ago, I did 6 months TAD for an OSO before going to flight school)
SkidDriver is absolutely correct concerning how the bodyfat is measured. Hydrostatic (i.e. being dunked in a water tank) is the most accurate method known, the electrical method is not far off. Even the caliper method is forbidden. The Corps allows none of the above: you do the tape measure method described above - and that is not done by a certified physician or physical trainer but rather a lance corporal in the Operations Dept. The Corps wants runners, not weightlifters.......
Sept 11 has really skewed the pilot numbers. As you can imagine, more than just a few aviators changed their minds about getting out. However, I believe we are still short of pilots.
Finally, if you don't get the particular aircraft you desire, there is the possibility of switching from helos to jets later in your career. Also, if you go reserves after your active duty commitment is complete, you can switch airframes (i.e. from a Phrog to a Snake) or even possibly stay in the training command flying T-34C's or T-45's (that is a new program because many Marines were joining Navy or Air Force reserve units due to location)
Good Luck and Semper Fi!