2 of 5 adding some background info

typically you want to power the ESCs directly from the battery, and use a BEC to power 5v electronics such as your controller. However, a lot of ESCs have the ability to provide 5v power back to the controller over the 5v line on their control cable (servo-style 3 pin cable). In that case, you wouldn't need to use a BEC at all, but it's possible you might have issues with more than one ESC all trying to supply power on the 5v line with them all hooked together. For maximum flexibility, it might make sense to select an ESC which has switchable BEC capability.

Edit:

Here's some of my previous quadrotor projects:

little quadrotor http://sphotos-b.xx.fbcdn.net/hphotos-ash3/27742_582181287582_1322230_n.jpg

http://www.youtube.com/watch?v=4eROXtMq4Q4

big quadrotor http://sphotos-a.xx.fbcdn.net/hphotos-prn1/32092_581531404952_6152279_n.jpg

http://www.youtube.com/watch?v=zV5HZIDCn58

I never got as far along as I wanted because I made them too big. The big one was the first one, and it was WAY too big; the smaller one was closer, but still too big. They had too much rotor inertia, and I couldn't spin them up and down fast enough for the PID control to stabilize.