So...i had the same issue with the candyshell not sitting flat, making typing tough without holding the device while it was laying on a desk. Tonight, I decided I was going to try fixing it, as I haven't had any luck finding another case I like better than this one...
and it worked! All it took was a heat gun set to low heat, wave it over the back of the phone for 5 to 10 seconds, and use a bit of force with a towel (don't want to lose any fingerprints) to push in the center for a couple of seconds at a time. Repeated the above steps for about 5 minutes, and now it's flat as can be.
The heat didn't harm either the inner nor outer shells, and I can now press down on the center of the case while it's on the phone, and you can't feel an air gap anymore. MUCH BETTER!