Microsoft has made Service Pack 2 RC available to the public today via their Customer Preview Program. It's avaliable to anyone interested in testing SP2 for Windows Vista and/or Windows Server 2008 prior to the final release.
You can download the Windows Vista and Windows Server 2008 Service Pack 2 RC from the Microsoft Customer Preview Program site or install through Windows Update using their Windows Update Experience Kit. Remember, you will have to uninstall previous versions of the Service Pack before installing the RC and you will have to uninstall the RC version before installing the final version.
You can find a full list of changes here and you can submit feedback on the TechNet Forums.
SP2 for Windows Vista incorporates new improvements discovered throughnthe automated feedback from the Customer Experience Improvement Programnin addition to all of the updates released since Windows Vista SP1. SP2nfor Vista will come in around 302MB in size for the 32-bit version andn508MB for the 64-bit version. Among the new features are Bluetoothnv2.1, Windows Connect Now an approved WiFi manager, support for exFATnor FAT64, and support for VIA's Nano CPU.