Our pediatrician gave me the AOK to have a nice cold brew while breastfeeding. He said the best time is immediately before a feeding--the alcohol can even aid in "let-down" for moms having trouble with that. If you're drinking a beer while your'e feeding, the alcohol won't have time to make it into the breastmilk, and the alcohol should be well metabolized by the time your next feeding comes around.
I'm one of those dames who really enjoys the taste of beer, so I took that green light to heart--but I never had more than one an eve while I was still breastfeeding. Mommy passed out cold on the couch is such a nasty sight, and I'm a lightweight :*}