Okay. Dr. v. Professor v. First name is legit. Never Mr. or Mrs. for someone who holds a MD or PhD.
I side with the NYTimes Style Guide and opt not to use Dr. for anyone that isn't a MD. At least not initially. If they hold a PhD and request it, great, I'm happy to oblige. I prefer, however, to address professors with "Dear Professor..." If asked/instructed to use the first name I'm happy to do that as well.
But to reiterate the major theme of the discussion, it is better to be overly formal and stay on the safe side.
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From Phillip Corbett in the NYTimes usage blog, "After Deadline."
http://topics.blogs.nytimes.com/2010/04/13/faqs-on-style/?scp=4&sq=style%20dr.%20mr.%20mrs.&st=cse
Who’s a Dr.?
Our continued use of courtesy titles — increasingly rare in the news media — prompts many questions. Rules on the use of “Dr.” in particular can lead to confusion, for readers and unfortunately sometimes for our writers. Here’s our stylebook entry:
Dr. should be used in all references for physicians and dentists whose practice is their primary current occupation, or who work in a closely related field, like medical writing, research or pharmaceutical manufacturing: Dr. Alex E. Baranek; Dr. Baranek; the doctor. (Those who practice only incidentally, or not at all, should be called Mr., Ms., Miss or Mrs.)
Anyone else with an earned doctorate, like a Ph.D. degree, may request the title, but only if it is germane to the holder’s primary current occupation (academic, for example, or laboratory research). For a Ph.D., the title should appear only in second and later references. The holder of a Ph.D. or equivalent degree may also choose not to use the title.
Do not use the title for someone whose doctorate is honorary.