As a college student, I've come to embrace Linux and the FOSS movement in general. I currently work at a small company which produces open source software for large-scale scientific computing.
As I look to graduate school and beyond, how can I prepare myself for a career in FOSS development, and what are some of the leading companies that contribute to the open source movement?
You'll find that most large FOSS friendly companies make money from alternative revenue streams.
However, you could go work for the Mozilla foundation, IBM, Redhat, BEA etc.
Just realize that they are all corporations in the end, and just because they are slightly more altruistic by giving away the source, doesn't make them any different than your typical soulless corporation.
Seems to me that the number one thing open-source projects look for in a developer is your portfolio, or the other open-source projects you've worked on. I would think you should start contributing to several open-community projects--the ones closest to the kind of project you want to work on.
You can probably find some good advice on Matt Asay's blog, which covers open source from a business perspective. You'll probably find this post (also reprinted here with additional comments) relevant.
Paul Graham might advise creating or joining a FOSS startup. Even if you fail, you'll get some great experience, and right before or after grad school is a good time to do it (no family or mortgage to worry about). Not that I've ever been bold enough to follow that advice.