I had my HD replaced under the Seagate recall program. Apple removes the defective HD and replaces it with a new one. As such, if there is any data on your exisiting HD you do not want others to possess, the safest thing to do is to securely erase the HD before taking in for service.
One way you can do so is:
1. Back up the HD completely.
2. Disconnect the drive containing your backup from your computer.
3. Use FileValult to encrypt each user account. When asked by FileVault, tell it to securely erase the "old" version of each user's Home directory. The secure erasure can take quite a long time (on my machine, a 260GB directory took around 24 hours and a 34GB directory took about 5 hours) , so don't do this right before your appointment to have your HD replaced.
4. If you want to be extra secure, create a temporary user account with Admin privileges and delete all the other user accounts (using the temp Admin account).