First of all, I am a Dental Hygienist, and have been presented with this question more times than I can count. It is totally false that a pacifier can cause an overbite IF you are using the orthodontic nipple type pacifier. If you notice on paci's, the round-type nipple has a much rounder, larger base that is between the child's teeth when sucking; whereas, the orthodontic nipples are basically flat at the base. Therefore, thumbsucking can cause a protrusion of the front teeth as well. However, most of this is caused by malformation of the palate. By sucking on something round, as in a thumb, or a non-ortho paci, you are shaping the palate (roof of the mouth) into a narrow, rounded high vault, thus causing the arch of teeth to become closer together and narrow, which results in the "buck-tooth" look. This can really be an issue later on, as most children require a palate expander that you have to crank at bedtime, and is VERY painful for the child. The orthodontic type paci's flatten into the roof of the mouth, helping to properly shape the child's palate. This is also why ortho paci's have an age guide line on the package that is very important to follow. The nipple increases in size with the age of the child, making sure that the palate is staying appropriate size and shape. That is the professional viewpoint of paci's.
Now, on to my personal viewpoint: I have a 3 year old who sucked an orthodontic paci until this past september, when we had our second baby boy. He didn't want to give it up, but we came up with the idea of the "Paci fairy". She came and collected all of his paci's (without him knowing) and left him a really great surprise in exchange. He was thrilled. Any time that he asked for his paci, we would say, the paci fairy left you your truck instead, and he was fine. It worked perfectly!!!! We were soooo excited! If you are ready to get rid of the paci, this is just an idea. But you have to make sure that the gift is really worth it to the child. Ha! Good luck, but don't stress over this! My son has beautiful teeth!