If you plan on being any sort of security professional, you need to know about more than just Windows machines.  Linux is a powerhouse in the network environment and its use will continue to grow.  There are plenty of ways to learn Linux.  For one, you can just go download any distribution and use it for a while.  Keep in mind though that the ones used in production environments tend to be the Redhat/Fedora, SuSE, and Debian distributions or derivatives.

HP has had this website up for a while but their TestDrive service lets you get an account and use one of multiple Linux and UNIX systems, open source AND commercial,  over the internet so you don't even have to do anything to your PC.  The good thing about this is you can jump right in and start using a *nix system and learning about it.  The bad part is you don't get the experience of installing it.  Which could be very easy with the mainstream distributions or very tricky with the less well-known things.  Regardless, you should take steps to learn these systems inside and out.

Defensive and Offensive security go hand in hand.  If you're defending a network, you better know your systems and how to secure and properly configure them.  If you're conducting penetration tests on a network, you better know how to exploit flaws in anything you can get your hands on, including the more obscure OSes like OpenVMS and HP-UX.

There is more than enough information in online forums, IRC channels, and mailing lists to give you the knowledge you need to learn these systems so get going.  If you're like me, you'll appreciate the challenge of any new operating system anyway.