How much current/power can be drawn from iPhone 30-pin connector?
Somewhere around 160 mA.

Kineteka sells a 30 pin breakout board and pin 18 supplies the 3.3 volts. I just connected a 500 ohm pot between pin 18 and ground and lowered the resistance while measuring the current. It tops out around 160 mA. Try it!

EDIT:

Kineteka has closed but here are some similar 30 pin breakout boards with which I have no experience: http://www.pc-mobile.net/iphone.htm