Although this certainly doesn't ring true everywhere, at my company we use Linux and BSD almost exclusively for our tools. Although I would never reject a candidate based on their distro of choice, being intimately familiar with linux and command line tools is a must. You'd be surprised how many people are confused by even simple tasks in other operating systems.
The most important thing you can do, however, is show that you're passionate about the subject. Do security related projects, stay current on news in the field, publish your work if you can. Attend conferences like DEFCON if you can, and if not at least try to catch videos of some of the talks.
The field is growing and loves to find new, bright, interested talent. It's hard to separate people with a real, keen interest in security and a background to support it from the kids that try to bullshit their way through, though. Be honest about your skillset, your interest level and your background, and it will all fall into place.
Awesome book covers EVERYTHING with practical instructions + a comprehensive checklist.