Folks,
You should really make sure to introduce yourself as Dr Lastname. Even if you dont' "feel" like a doctor yet, you are. It is imporant to establish yourself as a doctor. This is a credibility/authority issue.
Especially important for females. Even after you introduce yourself as Dr, there are a number of people for whom it wont' sink in. They will think you are a nurse, NP/PA (if there are a lot of them at your institution) or even the unit secretary. I just finished PGY 2 and still I am regularly mistaken for all of these, based soley on my gender. A common scenario last month, during rounds with the rest of (male) burn team in the ICU...A family member of a pt we weren't following would come out of that pts room and come up to me and ask, "are you his nurse" or state "he's ready to get back in bed now" or something like that.