Wow, there's a regular tribe of us with this problem. My (holistic) dentist said it would be okay to wait on mine, but then I had a consult with a lady in the office who helps you plot out your treatment and she said they should come out right away. Also, one of them has a cavity by it, so that one needs to be dealt with immediately anyway.
She also said it would be best if I could go 20 hours without nursing, and that I pump to give him milk and then pump out the milk after the procedure, just in case I take in any mercury. I have a 20 month old, and pumping is just not a part of our lives at this point, plus my pump is crap, so I don't know that I could even get anything out that way. Ds nurses A LOT, but it's for comfort, so a bottle or goat's milk isn't going to do it -- I'd basically have a screaming horror show on my hands if I tried not to nurse for that long. She also said they'd like to have me to a 3 hour Vitamin C IV to detox, even though they take good precautions. My hangup here is my aversion to needles. I can get through a quick shot or blood draw, but 3 hours with an IV will certainly result in a major panic attack.
Anyway, it all just sounds like a huge mess to deal with, and I'm on the verge of burying my head in the sand on the whole thing. Neither ds or I are showing any ill effects from my leaking filling. I don't know. They just don't really get what it is to have a toddler nursing. My holistic dentist was all shocked that he's still nursing.