1) tearing down houses and removing foundations is what I'd imagine
2) if this is even being considered, I'd guess it's for properties than have fallen into city/state hands due to tax deliquency
3) why not? there's actually a proposal about opening up vacant tracts of Detroit to commercial-scale farming. no reason why that wouldn't work elsewhere. having said that, in many places, 12"-18" of topsoin would have to be trucked in for some crops, due to contamination of existing topsoils.
4) farmland gets sold to developers all the time; this shouldn't be any different. if the reversion-to-nature goes so far as to remove/cut off water, gas and sewer lines though, that could run into some serious $$$
5) concentration/contraction into cities is inevitable at some point, unless enormous new oil fields are discovered...and even then, that'll run out someday