Erik--held out all of 1 week. He clearly had no nursing issues, and he went back and forth between them just fine. We took the paci away at 4 months, when his need to suck was decreasing. He was fine with it.
Anders--I waited a few weeks, and he did actually have issues going back and forth between paci and breast. He figured it out, but for the first several weeks, every time he'd nurse after using the pacifier, it took him a few seconds to latch on and start sucking right. He weaned himself from the pacifier at 7 months--just refused to accept it. It took me a few days to figure it out--I'd be rocking him to sleep, trying to put the pacifier in, he'd be spitting it out. LOL!
I was anti-pacifier before I had kids.

I am still anti-walk around and play with a pacifier in the mouth as a toddler/preschooler... but I don't get up in arms about it. I also know that every child needs some security on some days, and I don't make any judgements about people I see. just yesterday, in fact, my friend came to playgroup and her 17 mo old had his pacifier in. She said they both woke up in need of a little security (his twin sister was carrying her lovey blanket), and they both had their security for about the first 30 minutes, when they were able to set them aside. I thought it was great how she read their needs.
Oh, I didn't use a paci in place of comfort nursing ALL of the time. I still nursed for comfort and security. But there were times when the baby wanted to suck, but did NOT want milk. Especially in the first few months, I had milk a plenty, and it was hard for a baby to nurse wihtout getting milk.