I've been a Windows network admin for years now and would like to broaden my knowledge by learning Linux (I know...kinda behind the times but never had the chance in my company). I know it doesn't happen overnight, but I need help with where to start. What version/product is most common in corporate networks? Does it even matter, or is it if you know one you can basically administer them all?

Thanks

Welcome to LQ!

Everybody is going to tell you Red Hat Enterprise Linux is the one to learn for your purpose, and there is free RHEL clone CentOS.
I say get Debian or Slackware and do all administrative tasks from command line.

I am a complete newb with PCs(repair) and Linux...I have found that what is helping me is not being afaraid to ask any questions,as well as trying out the various iso Distro CDs..
Practice is helping me,however i am far far from perfect.

All distributions can be managed from the command line and the commands are largely universal. (Check out a book like "A Practical Guide to Linux Commands, Editors and Shell Programming"). The software is also largely the same. Where distributions diverge, however, is the format of software packages (think .exe equivalents) and the systems that are used to install/update/remove them (a.k.a. package managers). Red Hat, Fedora, CentOS, Suse and Mandriva use RPM packages and use yum, yast or urpmi as a package manager. The Debian branch (Ubuntu, Mint, Debian, Knoppix, Mepis, etc.) uses the deb format and apt-get or aptitude as a package manager. And then there are a handful of other formats but those are rarely if ever used in a corporate environment. All in all, if you invest some time in one, you'll find that it is quite easy to transition to another one.

Thank you all for the replies with good info. I'll keep my eye on this thread in case anyone else has more input or suggestions.

Yea ... there is not much left to say. You can use command line to manage every Linux box, most files you need to edit are in /etc and editable using any text editor. Some distros (like RH) have little GUI tools to edit those files. If you choose to go command line then you are not restricted in any way, you can ssh into a remote box, you can fix things if GUI does not start up ... a server does not need a GUI anyway, why waste resources, right?
Depending how much you know about POSIX systems you may find this piece of reading refreshing, annoying, insulting ... http://linux.oneandoneis2.org/LNW.htm