That question was presented to me recently. At first I was at a loss for a proper answer. What would you do if a potential job hung in the balance of your ability to answer that question?
Professionally, I worked with Unix for about five years on my previous job. My current job requires nothing more than Windows XP Pro. I have worked with Linux off and on as a hobby for about eight or nine years. Each time I pick up Linux, hack around for a while, then find a blinding limitation and drop it until next time. Currently my biggest problem with Linux is the lack of native support for the Broadcom BCM4318 wi-fi chipset, one of the most common in use today. As time goes on, I invest more time and money in applications for Microsoft Windows, which makes it harder for me to use Linux for anything BUT a hobby. Until Wine/Crossover Office get easier to use and support more Windows applications, I'm pretty much going to stay with Windows.
In any case, here is how I handled the question regarding my lack of professional experience with Linux:
"In my previous job, I worked with Unix on a daily basis for more than five years. My current job rarely requires the use of Unix, but I do occasionally play with Linux as a hobby, which I have been doing for almost a decade. Unfortunately, in my industry, it is very hard to work professionally with Linux, as my industry requires professionally developed and supported systems for mission-critical applications. Although some people tell me that Linux and Unix are entirely different, that has not been my experience and I believe I am able to move between the two easily."
I believe that answer satisfied the question. Feel free to use it if you need to, although substitute your own experience, as you can't use mine :-p .