That's a good link MaryJane posted above, and OniEhtRedrum's knew the answer, too. It's not that weed weakens the effectivenes of the Pill; it's that any kind of smoking in combination with the estrogen/progesterone in the Pill puts women at much higher risk of blood clots and strokes.

The Pill in combination with inhaled smoke makes women more likely to develop deep-vein thromboses (blood clots, usually in the legs) that can break loose and travel to the brain, lungs or heart. That's a bad deal when that happens, and it doesn't just happen in older Pill users. I had two friends in college who smoked grass and took the Pill and developed blood clots in their legs. Luckily, neither of them had a stroke and their docs took them off the pill when they developed sore lumps in their legs.

It's just not worth the risk because a stroke is the worst. If you want to smoke, it might be better to see about another form of birth control. Problem is, other most effective hormonal methods like the patch or the implant or the monthly or quarterly injection usually have the same no-smoking prohibition.