I picked up a 2012 JK last month and though I haven't had frost yet, she does fog up easily and inexplicably. Not every day, but some days I'll get out to the parking lot after work and it looks like someone's been gettin' hot and heavy without me... lots of fog on all the windows.
I've just assumed "it's a Jeep thing" and so far don't mind throwing the heater on defrost and letting her run long enough to clear up but if I didn't love her for so many other reasons I can see how it could get annoying. I have a model with the "A/C bypass option" (that is to say, no A/C) and, weirdly, there is no way (that I've found) to switch between outside air and circulating. I suspect that's the problem... in my other vehicles I've always found making sure it's set to outside air allows the difference in humidity between inside and outside to balance out and stops interior frost.
It's true, once the engine warms up the heater is almost hot enough to blister your skin (ok, not quite but it throws nice and toasty heat) so I imagine any frost that does accumulate will be no match for that.