RDP 7.0 protocol included with Windows 7 and 2008 R2 brings some nice enhancements (WMP redirection, bidirectional audio, multi-monitor support, Aero glass transparency etc), and now those features are available through the new Remote Desktop Client for Windows XP and Vista. See more below:
* Description of the Remote Desktop Connection 7.0 client update for Remote Desktop Services (RDS) for Windows XP SP3, Windows Vista SP1, and Windows Vista SP2
http://support.microsoft.com/kb/969084
* Remote Desktop Connection 7 for Windows 7, Windows XP & Windows Vista
http://blogs.msdn.com/rds/archive/2009/08/21/remote-desktop-connection-7-for-windows-7-windows-xp-windows-vista.aspx