I think I have touched on this before but here it is again. There is no need imo to put you footing below the frost line if you take care to prep the soil. I dug down 16 inches or so and 16 inches out from my slab and put good granual A gravel where my slab would be located. I have no worries.
Here is an article from Walters Forensic Engineering Inc. that may ease you mind.
http://www.waltersforensic.com/artic.../vol1-no11.htm
Cheers, Versachi