You can force protocol encryption. This forces all communication to be SSL encrypted. Therefore if anoyone sniffs the packets, they can't make any sense of the data.
There is some overhead in this - you need to deploy certificates to clients, set up certificates on the SQL Server. There is also proecssing overhead in encrypting and decrypting the data.
http://msdn.microsoft.com/en-us/library/ms189067.aspx