What if you turn on File Vault on your HD through System Prefs just before you take it in for repairs?
Isn't that supposed to encrypt your whole home folder so well that if you don't know the password the HD becomes a paperweight?
That would work, but personally I don't recommend that...
Because File Vault depends on the Operating System. The operating system might screw up some time (you might not be able to boot into OS X) and all your files will be gone. This happened to my friend. Her iBook became unbootable one day for no reason. All her encrypted data (by File Vault) were gone after re-installing OS X.
And similar situation happened to me once with Windows XP, and I lost all encrypted data what was encrypted by Windows XP's built-in encryption. Re-installing Windows XP did not help at all.