James Garfield (later president of the U.S.) was, in effect, a professor of Classics. He didn’t have a Ph.D. though. This was in a period when it was not so common for professors to earn Ph.D.'s.
Didn’t Teller of Penn & Teller have at least a masters? I’m having trouble confirming this, however. Wikipedia fails me.
Dan Grimaldi (Patsy Parisi) on The Sopranos has a PhD in Data Processing from City University of New York, according to his Wikipedia bio.
Cary Middlecoff was a praticing dentist when he quit to join the pro golf tour, winning 40 tournaments (two majors) and becoming a television golf analyst.
Frank Ryan, starting quarterback for the Cleveland Browns in the 60s, had a doctorate in math. He nearly passed on pro ball in order to get the degree, but managed to finish it while playing in a backup role with the LA Rams. After he got it, he was traded to Cleveland and became a starter. He led the Rams to their last NFL championship to date.
Author Stephen Coonts has a J.D. from the University of Colorado, and besides that was also a Naval Aviator, flying two tours of duty in the A-6 Intruder from the USS Enterprise during the Vietnam War (the basis for his first novel, Flight of the Intruder, which is totally awesome and you all should go read right now). While being a military pilot isn’t strictly speaking an academic achievement, I thought it was an achievement worth mentioning.
Stephen Hawking has a PhD from Cambridge, and of course we all know him from Star Trek: The Next Generation, where he played a holodeck simulation of famed physicist Dr. Stephen Hawking. He is also well known for his rap music 
[QUOTE=RealityChuck
Frank Ryan, starting quarterback for the Cleveland Browns in the 60s, had a doctorate in math. He nearly passed on pro ball in order to get the degree, but managed to finish it while playing in a backup role with the LA Rams. After he got it, he was traded to Cleveland and became a starter. He led the Rams to their last NFL championship to date.[/QUOTE]
Frank Ryan led the Browns, not the Rams, to their most recent championship way back in 1964. Kurt Warner from the school of hard knocks led the Rams to their most recent title during the 1999-2000 season. Its easy to get confused what with the Rams moving from Cleveland to LA to Anaheim to St. Louis. 
Two fantasy/cyberprep authors: Esther Freisner has a Ph.D. in Spanish and Susan Swartz has a Ph.D. in English from Harvard.
From the sports world:
Bart Oates of the New York Giants has a J.D.
Former Major League pitcher George “Doc” Medich is an M.D.
Ditto former Yankees 3rd baseman and A.L. president Bobby Brown.
Dexter Holland, lead singer of The Offspring, was a PhD candidate in molecular biology… he hasn’t finished it yet, as far as I know.
Are you sure? 
Weird thing is, her own Wikipedia page doesn’t make any mention of her going to NYU or getting a PhD, although biography.com says she does. (I have no idea how reputable a source Biography.com is)
Why the ?
-FrL-
While various sites say she has a PhD from NYU, I haven’t been able to find anything concrete. As Raguleader pointed out, her detailed biography at Wikipedia doesn’t mention the PhD. Neither does her bio at IMDB. They do mention an honorary degree from Wilson. (One site says it’s from Brandeis.) It’s hard to tell, of course, but from reading her bio I can’t imagine when she would’ve had time to get a PhD in literature. That would mean a BA + 6 more years in grad school. Her bio mentions she dropped out of high school, but nothing about an undergrad degree. I just don’t see when she would have had the time to earn a PhD from NYU. It doesn’t seem like something easily juggled with a career as busy as hers has been.
I was assuming the PhD was earned before her career took off.
-FrL-
Not a doctorate, but Ron Jeremy has a masters’ in special education.
Does a JD really count though? I know it’s called “juris doctorate,” but do lawyers really have to do a doctoral dissertation to get their degrees? I’m not pickin’ on lawyers, mind you, I’m just sayin’ …
According to Wikipedia, law students who do a dissertation instead are awarded a J.S.D., or a Doctor of Judicial Science, which is primarily for if you want to become a law professor or a legal scholar of some sort. Also, the American Bar Association requires schools to require J.D. students to complete “a substantial research paper of publishable quality” if the schools wish to be accredited with the ABA.
So, I dunno, but it sounds like they have to be a good bit more scholarly than I am. 
Also not a doctorate, but Danica McKellar co-authored a very well-regarded paper in mathematics, as an undergraduate.
She’s one of the few people with a finite Erdos-Bacon Number
Frylock writes:
> I was assuming the PhD was earned before her career took off.
There wasn’t any time before her career took off. Whoopi Goldberg had a child by the time she was 19. I think this business about her having a Ph.D. is a fake fact that gets copied over into new websites without anyone checking it. She may be quite intelligent, but I don’t think there was any time for her to get a Ph.D.
Typo Knig writes:
> Also not a doctorate, but Danica McKellar co-authored a very well-regarded
> paper in mathematics, as an undergraduate.
Let’s not exaggerate. It was a pretty good theorem, but it wasn’t anything earth-shaking. Incidentally, it’s a theorem in mathematical physics, not in math. She’ll have no problem getting into a top math or physics grad school when and if she decides to go for a Ph.D., but she’s not considered a great mathematician just because she co-authored a good but not great theorem as an undergraduate.