Amen! Barrels of beer are carbon sinks!
I concur and wish to back you up on your point that the C produced in fermentation is indeed part of the naturally occurring carbon cycle.
The best green beer would of course be made from more sustainably grown grains (using as best you can natural organic fertilizers, max-efficiency irrigation, etc.), and brewed using renewable energy for heating, mixing, etc. processes. I guess you'd need barrels made from sustainably managed forests, and oh--of course you'd need reusable/recyclable bottles or cans. Or just guzzle right from the tap.
Damn, I'm getting thirsty...