To coincide with the RTM release of SQL Server 2008 the SQL Compact Team have delivered Service Pack 1 for v3.5. This includes Entity Framework support and a native 64bit version. Steve Lasker has the details:-
http://blogs.msdn.com/stevelasker/archive/2008/08/07/sql-server-compact-3-5-sp1-released.aspx
Meanwhile SQL Server 2008 RTM is available already online for both MSDN and TechNet subscribers.
Disclaimer The opinions expressed herein are my own personal opinions and do not represent my employer's view in anyway.