I just entered DEP and will be enlisting as an MU. Laworlo is correct, it's an incredibly difficult program to get into and it's very, very small. My recruiting office had never processed an MU before and while at MEPS, I was informed they hadn't processed an MU in years. I took an audition in March and was put on a waiting list (at this time I had no obligation to the Navy and was not required to take the ASVAB or go through MEPS). In June I was contacted and told there was a position for my instrument open. THEN I went through the enlistment process- my ship date is in December. My counselor at MEPS told me they only allow one female MU at boot camp per month (and one male, I would assume).
You can find more info here-
http://www.public.navy.mil/bupers-np.../default1.aspx