Here's what kellymom says on the topic:
In short, this is the summary it gives at the end: "In general, if you are sober enough to drive, you are sober enough to
breastfeed. Less than 2% of the alcohol consumed by the mother reaches
her blood and milk. Alcohol peaks in mom’s blood and milk approximately
1/2-1 hour after drinking (but there is considerable variation from
person to person, depending upon how much food was eaten in the same
time period, mom’s body weight and percentage of body fat, etc.).
Alcohol does not accumulate in breastmilk, but leaves the milk as it
leaves the blood; so when your blood alcohol levels are back down, so
are your milk alcohol levels."
My MW also said a glass of wine was ok, but if you wanted to make sure it was gone before the next feeding, you would essentially want to drink while nursing. It wouldn't get into your milk fast enough for that feeding and should be gone before the next. She admitted, though, that many women would feel uncomfortable with a baby on the breast and a glass of wine in hand :-)