There is good news for you if you are nursing and would like to enjoy some alcohol now and then. Limited amounts of beer or wine are generally fine the American Pediatrics Committee on Drugs says that alcohol is a "Maternal Medication Usually Compatible With Breastfeeding". That doesn't mean you should be "self-medicating" with beer all the time of course. Nobody encourages alcoholism :). But a glass now and again is totally fine.
It is true that breast milk will be contaminated with alcohol if you drink huge amounts research shows that a bit less than two percent of the alcohol consumed by mom reached her breast milk. Drinking all the time in large quantities is always a bad idea, but for breastfeeding mothers, it could interfere with their let down, and can slow down baby's weight gain and development. If you are a social drinker who wants to join in a toast at a restaurant sometimes, you have no reason to worry.
Some experts advise not giving a baby milk that was produced within two hours of alcohol consumption. This means pumping and dumping the breast milk (See best tips for better pumping for more info on this). Others think this is not necessary if you are only having a glass or two. One thing that should be noted is that beer doesn't, like some old wives' tales claim, increase the milk supply. Large amounts of alcohol on a daily basis will in fact do the exact opposite.