Now that Vista is released Microsoft made the new Remote Desktop Client available for XP. This version is 6.0 and is basically the same client that ships with Vista. The killer feature for me is the /span monitors option. It allows you to create a remote desktop session that will fill two monitors. Note that the monitors need to have the same vertical resolution for this to be effective. Also they need to be oriented side by side. You do not need to have multiple monitors at the remote (server) end - just the client.
This one feature has been a real productivity booster for me. I have been using versions ripped from various Vista builds for over a year now, and can't live without it.
There are some other neat features in there as well, but most are limited for use with Vista as the server. These include 32bit color, font smoothing, and desktop composition. Oh, and by Vista I apparently mean Vista ultimate, since for some sick reason MS marketing yanked remote desktop support from Vista premium.
You can get the new client from windows update (if it hasn't already been pushed to you by now.)