Supposedly it's fine after the first trimester, but they prefer not to do it during the third because of the discomfort and strain on your vascular system from lying on your back for so long under the weight of your uterus. So, the second trimester seems like it would be the best time. It's a matter of risks vs. benefits and I don't think the risks are significant enough to outweigh the benefit.
Also, until you can have it done, try these things for pain relief, they helped my hubs enormously when he needed a root canal.
1. Sliced garlic, held between the cheek and the tooth in question. Yes, your breath won't be very pleasant, but you wouldn't believe the pain relief this will give you.
2. OregaDENT clove oil (found in most natural food/herb stores.) Ask your doctor about using this one during pregnancy, since I have no idea if it's considered ok. It's an antiseptic and an analgesic, so it will also help a LOT!
Best of luck!