Oops... I thought you meant a regular Arduino.
The BT is different, so you can disregard what I said about regulators above.
The Atmega chip specs should be the same.
I have a few BT's here, and here is what I can tell you:
TheDC-DC converter in the Arduino BT is a really really really teensy-weensy MAX 1676-EUB
. It puts out 300ma "typical", but maybe more, as the spec sheet says it has adjustable limiting, and we don't yet know how it has been 'adjusted' in hardware. I would guess 500ma.
The BT takes 1.5 to 5.5V as supply voltage. You can go down to .7V or so, according to the spec sheet for the converter.
Going over 5.5V will destroy the board.
There is a low-battery LED.
There is some kind of clamping diode on the input, but irregardless of this, apparently the board will die if you connect the power backwards.
The Arduino "9V" pin is actualy conected to the + battery supply. My half-dead batteries measure about 2.2V on that pin right now.
So, no 9V supply, just 5V at about 150ma(? after the BT module uses a bunch of juice) or so available for your circuits.
The good news is that the battery life is terrific with two AA batteries. (The radio range is also amazing, much farther than I would have imagined.) I have been running the same unit intermittently for several days on two AA Duracells. The converter will take every drop the batteries have to give, since it wil run down to very low voltages.