Interesting question... I assume you want your admin to back up the data. Which means the admin has to be able to read the information, but not be able to make sense of it... Which means encryption.
Fortunately MS SQL 2005 introduced comprehensive encryption support. I'll list some of the commands -- look them up for more help. If this doesn't work for you, you can use the (more comprehensive) encryption facilities in .NET, or use full-disk encryption for the database of concern.
Full-disk means, of course, you will probably be "outside" the normal backup support matrix. There are some "bump-in-the-wire" encryption solutions, that would work fine (but they are usually expensive, around 10 to 60K to start).
MS SQL ENCRYPTION COMMANDS:
CREATE MASTER KEY ENCRYPTION BY PASSWORD = 'password'
CREATE CERTIFICATE
EncryptByCert() and DecryptByCert() functions
CREATE ASYMETRIC KEY
EncryptByAsymKey() DecryptByAsymKey()
CREATE SYMETRIC KEY