+1
I had the scenario just a few weeks ago. Apple replaced my 1TB disk as preventive maintenance.
I just wiped my data directories before I turned the iMac in.
In case I cant' boot rom the internal disk I have an external FW800 boot drive clone (CCC).
And I have a "clean" administrator account (can give apple the password for troubleshooting).
For sensitive data I use an encrypted disk image. (
http://support.apple.com/kb/HT1578?viewlocale=en_US&locale=en_US)
The encrypted volume is of course fine for super sensitive stuff like banking passwords and bank statements and scans of your passport or driving license that you had to send to whoever.
Actually, I say it's fine, but actually it isn't. SSD's - because of how they do wear levelling and background garbage collection - are not secure devices. They leave copies of data hidden on the drive when they do their internal housekeeping and it is impossible to wipe this without doing a secure erase of the whole drive, which you can't do if the machine won't power up. So what about those files that you moved to the encrypted volume? Some are still visible on the SSD if someone really wanted to try.
And what about your photos? Or the letter you wrote to BMW complaining about their service? Or the video of your 3year old in her pyjamas opening her Christmas presents?
I am not sure i want any of that in the hands of complete strangers.
Ok, so you could encrypt the whole drive, but what does that do to performance?
On balance, I think it's just a whole load easier to install only a small SSD in your iMac and put everything else on an external drive. If I had a Mac with an internal fusion drive, personally I would unpair the SSD and hard disk and just use the SSD for the OS and apps.
Just my personal preference.