From my experiences camping, I can say that sleeping on hard ground (perhaps cushioned by a thin mat or piece of foam) is rather uncomfortable even for someone like myself who foam rolls, Mobility wods, and grok squats daily.
I've never given it more than a few days at a time (I'm usually rather sleep deprived, sore, and quite eager to return to my "normal" soft bed) but I would imagine that adaptation occurs over time.
There is no way that I could see that sleeping on a hard surface would "tone" the muscles. As another commenter said, the likely result would be greater muscular relaxation (and thus, improved posture, decompression of the skeleton and connective tissue, etc.)
I also think that this is something that shouldn't be jumped into too hastily. Just like the people who wear western style shoes and think that they can become barefoot/minimalist shoe runners overnight, most of us (myself included) are definitely not "in shape" for hard surface sleeping and a gradual transition would be the best way to do it without incurring unnecessary trauma.