Hey Katie,
Yes, you can get pregnant if you're periods have returned. You may have an anovulatory cycle or two, depending on your body and how often your lo is nursing. As long as your periods are happening, you certainly don't need to wean to conceive.
I've been pregnant three times now while nursing. I found that it was really hard to get pregnant when I wanted to. My doctor told me that the average time for conception is about 6 months after starting trying. There is quite a science to it if you're seriously trying, check out our TTC thread on the pregnancy forum.