Currently Being ModeratedApr 24, 2012 5:52 AM (in response to witenike)
I had the same problem. U need to install ITAP RPD, it supports spanning very well. U can test with their trial version.
I have a new IMac 27" with 27" Thunderbolt second display. Working on remote desktop with 5120 x 1440 pixels is awesome, and ITap supports it very well