Is it acceptable to ask how a professor would like to be addressed through email? I am an undergraduate student working for a professor this summer. Up to this point, I have referred to the professor as Dr. LastName. I have met his research group; his graduate and PhD students refer to him by his first name. Moreover, he signs his emails to me with his first name.

Is it acceptable to ask, through email, how he would like to be addressed? Or is it proper etiquette to wait for him to correct me? I would prefer to ask him in person, but I will not be seeing him for several weeks. 

For example, in an email: "I have noticed that you sign your emails as 'FirstName'. Would you prefer that I address you as 'FirstName'?"
In contrast to other answers voiced here, to me, signing a mail with {Firstname} is a clear indication that one can address the sender by {Firstname}. It would be unreasonable for the prof signing with {Firstname} to expect otherwise. I don't agree with the "always better to play safe"-advice, as I strongly believe in that an academic discussion should be based on arguments, not on status. If I expected my students to call me in a formal way, then I would have to address them in a more formal way as well (at least, that's what I feel). When I indicate that I as prof feel comfortable with the informal route (which I do by signing my mails in this way), there is no reason not to accept that offer.
Having said that, it is of course highly dependent on culture and language.