One of the bullet point features for Windows Vista Enterprize and Ultimate is the Unix subsystem for windows, which allows you to write posix... stuff? Anyway I'm outa my league talking about it... Anyone use this feature? Or explain it...
I know next to nothing about Unix programming, I'm interested in learning though.
It's probably best not to try to use the Posix subsystem for Windows. It was never really complete and is just a useless marketing tick box.
If you're truly interested in programming stuff for Unix, download one of the many Linux distributions (ie. Ubuntu) and VirtualBox. Install and start playing.
If you are interested in learning Unix programming, I'd recommend using Linux or FreeBSD. I haven't tried Vista's Unix subsystem, but past experience with Microsoft's Unix-compatibility stuff leads me to expect it will be a little "weird", and is therefore likely to confuse you if you are learning.
I don't want to discourage you from trying linux. But in this context it should be pointed out, that Linux is not completely posix compliant!
Wikipedia has a list of fully posix compliant operating systems
From that list, Solaris is probably the best to get started.
But anyway - for most of your posix-needs Linux should be the best choice (especially for beginners!)
The Posix subsystem in Windows is not only incomplete, but also slower in many cases than the "native" windows functions for the same thing. This is true for I/O for example.
In addition to Cygwin mentioned by another poster you should also consider MinGW.