whats wrong with using a perfectly stable os as a server? printer sharing..... check........file sharing.....check......
for a home server how much more do you really need?
I'm still toying with the idea of building a server out of my old Athlon rig - runnign bare metal VMware ESX, an OpenFiler image connected to a couple of large physical disks, and a smaller couple of drives to run a Linux distro to do torrents and whatnot.
I'm not saying you
can't or
shouldn't run XP as a server [it's fine, as you say, for basic stuff] but if I want iSCSI, rsync and NFS for VMs and backups, Windows ain't too hot for that - Solaris, BSD and Linux [and WinServer 200x if you don't mind paying for software to give you solutions for things that *nix boxes will do for nowt, like iSCSI, if you read the manuals etc!] are better choices if you are comfortable with them IME - you can literally fire and forget
Also please note that as a professional IT engineer [we could go into details about just how professional I am, but lets not...

] I am
LEGALLY OBLIGED to point out that anything by MS as a server is either a bit of a compromise or an utter joke. It's not that I agree with it, it's just the law
[but yes, fair point about XP doing the job for basic stuff, and I was being slightly drunkenly fascetious with that comment - but as an engineer, at home I need more than XP can offer out of the box and my experience of supporting NT4/5/6 based server systems has been more painful than anything with an 'ls' command in it to date...]