The Friday beta includes an update to QuickTime X that enhances reliability, security, and compatibility. The release also provides performance-enhancing tweaks for Apple's 64-bit Logic Pro audio suite, better compatibility with a host of printers, and applications that use OpenGL.
Apple is asking developers to evaluate other core system components in their testing, including VoiceOver, AirPort, and graphics drivers. There's a known issue with the beta when viewing photo albums using Front Row, but the approximately 670 MB release should arrive for the rest of us within the next six weeks.
Although past performance is not necessarily an indicator of future releases, both 10.4.3 and 10.5.3 were released about 110 days after the earlier versions. That would put the release of 10.6.3 near the end of February, 2010.