http://www.winsupersite.com/faq/xpsp2.asp
Q: What happened to the concurrent sessions feature in XP SP2?
A: In February 2003, internal Microsoft documentation described a feature planned for XP SP2 called "concurrent sessions." This would have enabled XP Professional Edition systems with Fast User Switching (FUS) enabled (i.e., nondomain systems) to support two concurrent interactive users. The current XP version is limited to one interactive user at a time; this user can be sitting at the XP machine locally or connecting to it remotely through Remote Desktop Connection (RDC), Microsoft's desktop version of Windows Terminal Services. Under the original plan for XP SP2, XP Pro would have supported two users, one local and one remote. This capability would have accomplished two goals. First, it would further differentiate XP Pro from XP Home Edition (an ongoing concern in Redmond) and make the more expensive XP Pro more enticing to users. Second, this feature would make Smart Displays more functional; under the current scheme, when a user accesses his or her XP Pro desktop from a Smart Display, the local system is logged out. Microsoft CEO Steve Ballmer, responding to complaints about Smart Displays, had promised that the company would add concurrent sessions functionality to the product in the future; XP SP2 was one way to accomplish this goal. But things change. Microsoft removed the concurrent sessions feature from XP SP2, at least temporarily, and current builds do not include this feature. It remains to be seen whether concurrent sessions will be included in XP SP2.