VIsta SP2 : preview

As is normal with all service packs, SP2 for Vista will be a roll up of all previous fixes, updates and applications.

Microsoft also states on their site that:

Application compatibility improvements

Thanks to the rich instrumentation capability of Windows operating systems, we are able to understand the types of issues that our customers experience while respecting their personal information and privacy preferences. We use this information to focus improvements in Windows, but we also share this information with our software vendor partners to help improve the reliability and compatibility of non-Microsoft applications. For example, in Windows Vista SP2, Spysweeper and ZoneAlarm, now work with POP3 e-mail accounts.

There is also this:
Hardware ecosystem support and enhancements

* Adds support for the 64-bit central processing unit (CPU) from VIA Technologies, which adds the ID and vendor strings for the new VIA 64-bit CPU.
* Integrates the Windows Vista Feature Pack for Wireless, which contains support for Bluetooth v2.1. Bluetooth v2.1 is the most recent specification for Bluetooth wireless technology and Windows Connect Now (WCN) Wi-Fi Configuration.
* Improves performance for Wi-Fi connection after resuming from sleep mode.
* Adds new capabilities to Direct X Graphic display reliability. The graphics experience has been greatly improved for users, such as gamers, running applications that require a lot of graphics processing.

There are additional features listed by Microsoft that also will be included with SP2 for Vista. It appears that SP2 final may be ready by April, 2009. But that us purely a guess.