There aren't a ton of studies investigating the effects of marijuana on breastfeeding babies, but we do know that THC (the part of weed that makes you high) does pass through your milk to baby. It is metabolized much more slowly than alcohol, with a half-life (the time it takes your body to get rid of half of the amount in your system) of 20 to 36 hours.
In one study, results suggested that THC in breast milk during baby's first month could lead to decreased motor development at one year, but there aren't really any studies showing more long-term effects. In the short term, pot-smoking moms have reported lethargic babies that fed less frequently and for shorter periods after exposure to THC in their milk. Plus, animal studies have suggested that marijuana can lower a mom's milk supply. (There haven't been any human studies on this front.)
So basically, nobody really knows how bad THC exposure is for baby, but pretty much all experts agree that it's bad idea.
There are a couple of other problems when it comes to marijuana use. For one, weed (or any drug) can seriously affect your judgment and ability to take care of baby. And, for another, you don't always know what's in your joint. It's common for dealers to add extra ingredients to street drugs to stretch their stash (more product = more money). It's impossible to know what those ingredients are, or what their effect will be on you, your milk, or your baby. So we say steer clear.