So, seeming how I always complain about the lack of multimonitor support in everything everywhere, you would have thought I would have seen this earlier (Seth pointed it out to me a couple of weeks ago).

 In Vista, typing mstsc /v:[terminal server name] /span in the run box, the span flag lets you "span" remote desktop across multiple monitors. It is only psuedo multimonitor support, since the server treats the whole viewable space as one monitor, but it is definitely much better than it was before. You could pretend to span before, but the buffer size really limited less than 2048 across (it now goes up to 4096x2048),

There are some rules involved, check out the whole post on the Terminal Service Blog entry. This change (and some others) were also back ported to Windows XP and Windows 2003 server with the release of Remote Desktop Client 6.0