I just came across a useful article about Vista remote desktop supporting multiple monitors.

Vista Remote Desktop Wide Screen:

You can access these features using the command line in a few ways:

If you know the dimensions that you want, you can specify the height and width:

mstsc /w:2048 /h:768 /v:(servername)

or if you want mstsc.exe to figure-out the dimensions:

mstsc /span /v:(servername)