I got a double major in Math and Computer Science and went on to get a PhD in Cryptography
(a discipline that sits between mathematics and computer science) at MIT. I currently
work as a scientist for BBN, a company in Cambridge, MA that does research and development
work for the U.S. government (mostly DARPA).
The bizarre thing is, I really have no clue whether I "use mathematics" in my career. I work on a variety of projects, but I spend a lot of my time analyzing network protocols to see if they are secure (and recommending changes in the cases where the protocol security is somewhat suspect). So I don't have a job that requires any use of calculus or linear algebra. I do use number theory at times, but the most valuable thing that take from my undergraduate mathematics background is a way of thinking. That is, mathematics has taught me to reason about abstract logical systems, and ultimately a computer network protocol is just an abstraction that allows one to logically analyze the arcane voltage fluxuations that occur on a wire connecting computers.