Have you tried booting off of an external drive and then looking at the Fusion disk using Terminal? Maybe you could start the partition process again by using that process. Here's an article that discusses using Terminal to make a DIY Fusion drive, but it may be applicable to you.
http://macs.about.com/od/diyguidesprojects/ss/Setting-Up-A-Fusion-Drive-On-Your-Current-Mac_3.htm
Right ... I seriously doubt there is anything
physically wrong with this iMac. The disk format is screwed up due to the
terrible advice given him by the Apple Care Tech, but that can be fixed without returning the unit. Unfortunately, the Apple Store Genius techs have no clue how to re-format these drives ... they need some training ASAP. Using 3rd party Disk Repair Tools probably wasn't good either as they probably don't understand Fusion technology yet and need to be updated.
There is a good chance that
Disk Utility (the one shipped with the new Macs) might even
fix it automatically, if he could just get into the Disk Utiliy program (I believe the iMac is non-bootable at this point). Otherwise, a simple DIY Fusion re-create would probably restore it to working order in a matter of minutes.
If the OP can still access the
recovery partition (command-r) on boot, this would give him access to both
Disk Utility and
Terminal to attempt to restore the original Fusion drive. Otherwise, either a net-boot or external bootable OS X (if he has one) would do as well. The hidden recovery partition on a Fusion drive is located on the hard disk, and thus may have been damaged as well with the aborted re-partition attempt.
It is unfortunate the OP has been put through all this grief and stress as the Apple Store should have simply known how to do this for him ... but they didn't.
A simple exchange would have also been prudent, but there aren't any available at this time, and replacement orders are lengthy, but improving.
I would have tried this simple restore process a long time ago and avoided all the hours dealing with it.
