At the Unknown Alma Mater, some of the faculty preferred that students address them by their first name. With others, it was "Dr. So and So" until they graduated. At my current school, american PhD students go by first names, and international students go by "Dr. ___" (at least, until I can break them of the habit).
For students looking for some guidance on the issue, PhD Comics provides a handy flow chart. Enjoy.