SQL Server 2008 will have a mechanism to encrypt the database at rest, with little overhead, and you can try this in CTP5. While it is on the instance where it was encrypted it can be accessed by any application with out modification. However
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Read the complete post at http://blogs.technet.com/andrew/archive/2007/12/21/sql-server-2008-transparent-data-encryption-and-replication.aspx