This http://blogs.technet.com/dataplatforminsider/archive/2008/04/15/sql-server-2005-sp3-coming-soon.aspx is the first public announcement I have seen which confirms service pack 3 for SQL Server 2005 is on it's way.  There is also some really good information appearing on the PSS blog http://blogs.msdn.com/psssql/archive/2008/04/25/sql-server-2005-sp3-and-self-service-hotfixes.aspx about the SP3 build and CU hotfixes. 

One of the more frequent requests I hear is for a slipstreamed build (downloadable image or DVD) which includes the service pack, or at least the ability to apply an integrated build.  Hopefully this may happen sooner rather than later.