I’ve just spotted SQL Server 2005 SP3 was released just before Xmas. It doesn’t appear to be a major pack (like SP2) so I would only advise installing SP3 on fresh installs. Upgrading will only be critical if the service pack addresses a specific issue on existing servers. Any 2005 server not on SP2 should be upgraded at least to SP2, as i’ve noted before.
Full details here: http://msdn.microsoft.com/en-us/library/dd353312(SQL.90).aspx