Re: OS to know.

[kmuskrat () gmail com]

You'll definitely want to learn more about network security as a whole.  You can know a lot about how to use an 
operating system which is popular and have no clue about security. 

Further, most network/computer security concepts are the same among operating systems.  The concepts of effective 
firewalling, intrusion detection, good practices, pen-testing, and so on will help you out a lot.  Once you understand 
the actual concepts, it's easy to implement such things in a number of different environments.

Like a few others said, learn all of them...or as many as you can.  If you simply focus on the ones that are popular, 
odds are you'll be competing for the job with a bunch of people who focused on the same thing.  If your knowledge is 
varied, you'll have an edge in the job market, especially if you happen to know an OS that isn't popular, but happens 
to be used at the site you're applying for.