SQL Server 2008 - Transparent Data Encryption (TDE)

Transparent data encryption (TDE) performs real-time I/O encryption and decryption of the data and log files. The encryption uses a database encryption key (DEK), which is stored in the database boot record for availability during recovery. TDE protects data "at rest", meaning the data and log files. It provides the ability to comply with many laws, regulations, and guidelines established in various industries. This enables software developers to encrypt data by using AES and 3DES encryption algorithms without changing existing applications.

http://msdn.microsoft.com/en-us/library/bb934049.aspx

http://msdn.microsoft.com/en-us/library/cc278098.aspx