Is available for download – http://www.microsoft.com/downloads/details.aspx?FamilyID=ae7387c3-348c-4faa-8ae5-949fdfbe59c4&displaylang=en
Change list is also up – http://support.microsoft.com/?kbid=955706
SP3 contains all the fixes from CU 1 to CU 9 as well as a few new ones. It’s possible to apply SP3 over CU 10 or 11, but then SP3 CU1 also needs to be applied to get up to date.
Update: CU1 for SP3 is now available. It contains only the hotfixes that were part of CU 10 and 11 for SP2. http://support.microsoft.com/kb/959195/en-us