I have Windows 7 at home, and routinely use RDC to access my XP PC at work. I have dual monitors and was told by one of our Sys Admins that I can use the /span option to take advantage of my dual monitors but it was not working for me - I was only getting my RDC on one desktop.
After verifying the requirements etc, I discovered the problem and solution. Windows 7 and the NVidia control panel disagreed on which of my monitors was primary and which was secondary. I changed it so that my DVI on the left was primary and my VGA on the right was secondary in both Windows and the NVidia control panel, and now it is working.
mstsc.exe /span rdc dual monitor not working not spanning span does not work remote desktop