More news has surfaced regarding the issue of having concurrent sessions under SP2 of Windows XP when operating Fast User Switching (FUS). This will allow more than one user to remotely connect to a machine at the same time.
A Microsoft proposal document on this matter outlines a number of scenarios that this will affect but does not specifically state that an active user physically logged on at the machine will be unaffected by a remote session. It does state that an existing remote user will be unaffected by a second session and seems to hint at the use of both a local and remote session at the same time, although in theory there should not be a distinction between a local and remote session.
If two sessions are already logged on the remote user may be given an option to select which existing user they wish to send a disconnect request to. Alternatively, if two remote sessions are already active, the disconnect request will most likely be sent to the session that is idle longest.
This is achieved by a registry tweak to change the MaxInstanceCount for Terminal Services under Windows XP.
Currently, it seems that the two concurrent sessions applying only to machines with FUS is still open.
Update: Microsoft asked us to remove the document supplied.