SQL Server Compact Edition RTM available
SQL Server 2005 Compact Edition extends the SQL Server Mobile Edition technology by offering a maintenance free, compact embedded database for single-user client applications for all Windows Platforms including Tablet PCs, Pocket PCs, Smart Phones and Desktops that supports in-memory processing. In addition, SQL Server Compact Edition is free to deploy and free to redistribute.
Check out http://www.microsoft.com/sql/editions/compact/default.mspx for more info.
There is a nice whitepaper that discusses when to use Compact Edition and Express