My two cents:
Marking as solved is no problem; the OP can still reply to the thread.
Moving something to the "solved" subforum is no issue for me personally, as I follow the "O HAY, new posts here" icons from the front page to find new & updated threads. Looking for an updated thread in "solved" should be pretty intuitive, but I can see it as being easier/more intuitive for some users to just go find the thread in the subforum where they originally posted it.