Microsoft has released SQL Server 2008 Service Pack 2 Cumulative Update 1, which is Build 4266. I count 67 fixes in this Cumulative Update, which is meant to get the SP2 code branch in synchronization with SQL Server 2008 SP1 CU11. This means that you can migrate to the SP2 branch if you had previously installed SP1 CU9 or greater (by installing SP2, and then installing SP2 CU1). The chart below might make it a little more clear.

-- SQL Server 2008 RTM is considered an "unsupported service pack" as of April 13, 2010
-- SQL Server 2008 RTM Builds     SQL Server 2008 SP1 Builds     SQL Server 2008 SP2 Builds
-- Build       Description        Build        Description         Build     Description
-- 1600        Gold RTM
-- 1763        RTM CU1
-- 1779        RTM CU2
-- 1787        RTM CU3    -->      2531        SP1 RTM
-- 1798        RTM CU4    -->      2710        SP1 CU1
-- 1806        RTM CU5    -->      2714        SP1 CU2
-- 1812        RTM CU6    -->      2723        SP1 CU3
-- 1818        RTM CU7    -->      2734        SP1 CU4
-- 1823        RTM CU8    -->      2746        SP1 CU5
-- 1828        RTM CU9    -->      2757        SP1 CU6
-- 1835        RTM CU10   -->      2766        SP1 CU7
-- RTM Branch Retired     -->      2775        SP1 CU8          -->  4000       SP2 RTM
--                                 2789        SP1 CU9
--                                 2799        SP1 CU10
--                                 2804        SP1 CU11         -->  4266      SP2 CU1            

You should be planning on moving to the 2008 SP2 branch sometime during 2011, since Microsoft will eventually retire SP1 just like they retired the RTM branch on April 13, 2010.  Also, make sure you understand that this CU is only for SQL Server 2008, not for SQL Server 2008 R2 (which is considered a separate major release).