Dang, got me!
I'm really just looking for a new learning curve to climb in order to maintain relevance.
I would recommend learning Windows first. It is by far the easiest, and most prevalent of all operating systems. ~90% of all desktops, and ~70% of all servers. You can get an evaluation copy for free that lasts for 180 days before having to be reinstalled, all you have to do is register:
Microsoft Windows Server 2003 Evaluation Download
I'd steer clear of any Unix or Linux product unless you feel forced into it, they just aren't as widely used, and a heck of a lot harder to understand much less master. If you do feel the need to take the Unix plunge, I'd start with Apple first, then Solaris. Good Luck.