This is from MicroSoft KB Article 925876:
The Remote Desktop Connection 6.0 client can be used to connect to legacy terminal servers or to remote desktops as before. However, the new features that are mentioned in this article are available only when the remote computer is running Windows Vista or Windows Server 2008.
MORE INFORMATION
The Remote Desktop Connection 6.0 client update contains the following new features.
Server authentication
Resource redirection
TS Gateway servers
Terminal Services Remote Programs
Monitor spanning
Visual improvements
I have not tried the new RDP TSC 6.0 so can anyone confirm if Monitor Spanning works on a RDP connection from computer running Win XP SP2 to another computer running Win XP SP2?