From the documentation: http://code.google.com/p/arducopter/wiki/AC2_ESC

4.You will hear a series of beeps. Now drop the throttle to full down. You will hear a different series of beeps. You may now disconnect your LiPo. Your ESC is calibrated. 

I think this #4 needs to be rewritten for the newbies, as a start something like:

Turn on the transmitter. Now drop the throttle to full down.

After plugging a LiPo battery, a loud non-smooth series of beep are heard with a bit of silence in between (beep+silence about 1sec), then a melodic softer 'tune' of 5 smooth lower volume beeps heard with no silence in between, a longer period of silence and then loud non-smooth beeps are heard again. The latter is the end of ESC calibration.

NOTE: Smooth tunes of less than 5 notes are heard in some occaisions. 

I will try to recalibrate several times to better this description.

D