Microsoft has released a Community Technology Preview (CTP) for SQL Server 2008 SP1. This is build 2520. My understanding is that SP1 includes all of the fixes that are in CU3 for SQL Server 2008.
Microsoft does more regression testing on Service Packs than they do on Cumulative Updates. Cumulative Updates are released every eight weeks, so you get fixes much more frequently than you do with Service Packs. Many DBAs and organizations still like to wait until SP1 comes out before they deploy SQL Server, so they should be feeling more confident soon.