Linux not, but OpenBSD yes...
My OpenBSD "server" is a file and application sharing server for Windows desktops, NT domain controller, NT domain logon server, roaming user profiles server, it also does internet connection sharing, Minitel sharing (Minitel is a french services network for low-end terminals which was a direct concurrent for internet in the 80s, but still some services in France are only available through Minitel), fax server, DNS proxy, firewall, printing server, and I'm currently wondering if I could not put a HTTP server on it as well that would host a certain site of which the idea to put it online has been roaming in circles in my mind for a long time already.
I have just upgraded it to OpenBSD 3.4 (I finished with the NT domain controller setup yesterday) but before this, I had not rebooted it for MONTHS. I swear only the power shortages brought it down for limited periods of time. Rock solid. Always did its work, and it always did it well.
Since the upgrade, the machine is now a Pentium 133 with 48 Mbytes of EDO RAM and a 15Gb HDD. Before the upgrade, the machine was a Pentium 90 with 32 Mbytes of SIMM RAM and a 1.2Gb HDD (without the X-Windows server, the OS takes less than 200 Mb).
Although I defy any penguin on this planet to achieve such a level of robustness, I imagine a well-configured Linux would come close, and I'm pretty sure it will replace advantageously any Microsoft server.
*edit*
For Windows services on UNIX machines, check out
http://www.samba.org
http://us1.samba.org/samba/ftp/docs/...PDC-HOWTO.html (outdated but interesting)
http://ps-ax.com/samba-fq.html (outdated too, not many recent docs on the new Samba 3.0.0)