Windows Server 2008 is definitely
not a prototype -- It is Microsoft's newest offering to enterprises and is very stable. The posted link above just gives instructions on how to make the server more usable as a workstation. Search around and you'll find that many people are happy with it. I haven't encountered any bugs, but keep in mind there are a
few features from Vista that are not yet working.
Permanent installation? Microsoft's prices are from
$999-$3999. There are ways to get around the 8-month limit, but I'll leave it up to you to find those if you feel comfortable with that... I'm just planning on using the Standard edition for 8 months and then trying the Enterprise edition for another 8, perfectly legal. By the end of 16 months I'll upgrade to another computer.
If you have a need to run Windows programs in Linux, you should definitely look into
VirtualBox. You can install Windows Server for free and run your Windows applications
seamlessly on your Linux desktop. Or just use it to try Windows out. Having used Linux for 6+ years, I understand the appeal behind open source software, but it's awfully fun to run one operating system inside another. If you want to run Ubuntu seamlessly on Windows, check out
andLinux.
Let me know if you need any help. Still running 2008 and it works beautifully!

EDIT: Note that Server 2008 is compatible with Vista drivers, as they share the same kernel.