I'll see what I have around here, dig through my archived IE Favorites links, etc. and let you know.
Here's what I think I know about it:
There was a former DEC guy on the OS/2 project who left about when IBM took OS/2 over.
He got the rights to the source to NT 3.51 to create the original Citrix products for 3.51.
Microsoft built their own Terminal Server version of NT4. Citrix stuff was mostly "value-adds" - like clients for non-Windows platforms, their own ICA protocol that is more optimized for WAN use - RCP kinda stinks over slow-speed networks.
MS integrated the Terminal Server stuff into Win2000 server (various editions, might not be in Win2K Server Standard, but is in Advanced and above). Citrix is even MORE of a kit of value-adds and less of an OS modification. But they still have the misc. clients and ICA protocol, along with some administrative helper stuff.
MS kept Terminal Server in WinXP (in beta now) but it is even in WinXP Pro in a limited, single user form as Remote Desktop Sharing and the Remote Desktop Assistance feature. This may even be in the Home version of WinXP when released. Full Terminal Server is probably in the server versions of WinXP too, but I have no direct experience yet.
The info above is imperfect, I know.
Luck!