SQL Server 2008 includes Transparent Data Encryption
Transparent data encryption (TDE) performs real-time I/O encryption and decryption of the data and log files.
Encryption of the database file is performed at the page level. The pages in an encrypted database are encrypted before they are written to disk and decrypted when read into memory.
TDE 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.
Check out a screencast about the new Transparent Data Encryption feature in SQL Server 2008 via MSN Video below, or if you prefer YouTube or download.