Windows Vista SP2 includes all of the updates that have been delivered since the release of Windows Vista SP1, and incorporates improvements discovered through automated feedback from Microsoft's Customer Experience Improvement Program (where users voluntarily opt-in to sending Microsoft error reports from their systems). In addition to these updates, SP2 also includes support for new types of hardware and emerging standards that will grow in importance in the coming months.
Windows Vista SP2 mainly adresses application compatibility issues, adds support for new hardware, features enterpise improvements (ie Hyper-V virtualization environment fully integrated in Windows Server 2008) as well as other improvements erlatd to setup and deployment. SP2 also contains improved support for Bluetooth devices, an updated Windows Search tool, support for Blu-Ray devices and a host of other tweaks. For specific changes in Windows Vista SP2 check out the notable changes document.
Microsoft is tracking to ship Windows Vista SP2 in the first half of 2009.