A while ago I noticed that quite a number of recent-ish philosophers did not actually receive B.A.s in philosophy--or whatever their equivalent undergraduate study was. So, I scoured Wikipedia (and the related internet) to compile a list of all the eminent philosophers who fit this criteria. The number is surprisingly high--or, perhaps I should say, the number of very influential philosophers on this list is quite high. I am certain that there are many, many more philosophers in general who did take philosophy as their first college degree, but if we only count only those philosophers with a very dramatic influence, I think we would find comparable numbers between those who began with philosophy and those who began with something else.
This list is unfortunately quite biased in favor of analytic philosophy, since that is the area I am most familiar with, although I did try to include some philosophers from other fields. I looked for philosophers from the end of the 19th century on--Nietzsche may be stretching it a little, but oh well. I also included a few notes about later/higher education, when it deviated from philosophy.
All sources are from Wikipedia unless noted otherwise. The list is in no particular order, although I did clump some names together based loosely on association or contemporaneity.
What was their focus as an undergrad?
Friedrich Nietzsche: philology (he later received a doctorate in this same subject)
Gottlob Frege: mathematics and physics
Bertrand Russell: mathematics (although he did study philosophy as an undergrad too)
Alfred North Whitehead: mathematics (also taught as a professor of mathematics for some time, before eventually becoming a professor in philosophy)
Ludwig Wittgenstein: mechanical engineering (until he came across Frege and Russell)
Karl Jaspers: medicine (received an M.D. and studied psychology. Did not study philosophy until he was 40)
Edmund Husserl: mathematics (followed by a PhD in the same subject)
Karl Popper: medicine? (I can't find anything that talks about his undergrad education, but he he received a PhD in Psychology, and this was early enough that he probably went to med school)
William James: another M.D. (Also did not study philosophy until he was older)
Thomas Kuhn: physics
Rudolph Carnap: physics (although he also studied logic and Kant's Critique). He originally intended to write a physics thesis--but both the philosophy and physics departments reputedly rejected his it as being too much like the other subject. He eventually wrote a more philosophically oriented thesis, so he ended up with a degree in philosophy.
Charles Sanders Peirce: chemistry (followed by an M.A. in chemistry too; source: SEP). In all fairness, it seems he also studied philosophy on his own time, and the reason he continued with chemistry was largely to support himself while he pursued logic.
Keith Donnellan: math education (source: biography on his site)
Max Black: mathematics. (I think, anyway. The only source I could find is a little unclear about it--and it does say that he talked much with many other budding philosophers of mathematics; source)
Saul Kripke: mathematics
David Chalmers: mathematics and computer science (afterwards studied for his PhD under Douglas Hofstadter (who is something of a computer scientist or cognitive psychologist--not a philosopher per se, but he is a very philosophically-minded polymath))
Otto Neurath: mathematics
Nancy Cartwright: B.Sc. in mathematics (source: cv from her site)
R.M. Hare: Classics (source)
Martha Nussbaum: theater and Classics
Jean Baudrillard: German (and later received a doctoral degree in sociology)
Arthur Danto: art and history
Hartry Field: mathematics (source)
Very interesting, is it not? What I find particularly fascinating is that there seem to be many more "crossovers" from non-philosophical fields among those philosophers who wrote on metaphysics or the philosophies of science, mind, logic, or math--rather than, say, philosophers who are known principally for their work in ethics. And that's not all. Observe the following list of double (and more) undergrad majors:
W.V.O. Quine: mathematics and philosophy
Frank Jackson: mathematics and philosophy
Hilary Putnam: mathematics and philosophy
Jaegwon Kim: French, mathematics, and philosophy
Ernst Mach: philosophy, mathematics, and physics (followed by a doctorate in physics).
Imre Lakatos: philosophy, mathematics, and physics
Carl Hempel: philosophy, mathematics, physics (source: IEP)
Paul Churchland: philosophy, mathematics, and physics (Source: c.v. from his website)
Philip Kitcher: mathematics and history & philosophy of science (source: c.v. from his website)
Nick Bostrom: philosophy, mathematics, mathematical logic, and artificial intelligence (source: cv from website)
Notice any, ah, glaringly obvious trends there? So, if you're an analytic philosopher interested in something beyond ethics--why aren't you brushing up your math skills right now? Some physics surely couldn't hurt either.
(I should note that a few other relatively frequent double majors showed up--such as law, economics, political science, and the Classics--but these were largely dwarfed by the majors in mathematics. This may be due to my afore-mentioned analytical philosophy bias, but I find it very noteworthy nonetheless.)