http://blogs.msdn.com/sideshow/archive/2008/06/02/sideshow-on-windows-mobile-released-techready-2008.aspx
This enables you to use a Windows Mobile 5 or 6 device (Standard or Professional) paired via Bluetooth with your Vista PC as a SideShow client device.
I tried this last night with my Okta/HTC Touch, and was able to use the device as a remote control for Media Player.