Not exactly true, since with the UNIX and all you can do programming in Java, C++ or Objective-C, Python, Perl, and myriad other languages. Since you can host web sites from your computer, you can get familiar with the Apache web server, write your own CGI, and use php and mySQL and other good stuff like that. And of course if you dick around with the Terminal, a lot of the UNIX stuff can transfer over well to Linux or any other *nix system.
Trust me on this; I've done it all myself.
That being said, though, it bears mentioning that most companies do use Windows, so there's a lot of stuff you won't be able to get experience with: IIS instead of Apache, C-Sharp or whatever the hell MS is making you write Windows apps in now, Microsoft SQL Server, Access, and plenty of other stuff I'm not familiar with since I'm a Mac user.
So, basically: If you want to go into a computing field, you can find an equivalent (often open-source) on the Mac for anything -- programming, databases, system administration, etc. -- you might do on the Windows side. But you won't be able to get familiar with the exact software that people use on Windows, and from what I've seen a lot of people look for that.