I think it will be very rare for high draft picks to go unsigned in the current system. The cost to the team of going over the cap is prohibitive, I doubt that ever happens. And no organization wants to not get their top pick. For the players, no player wants to miss his payday, whether it's a junior who's have reduced leverage next year, or a HS player who'll need to wait years. I think the pressures to come to terms are mutually reinforcing.
The main action takes place, I think, before a draft pick is made. The team knows what it can do, the player says what he'll need, and the team decides from there. I think that's why all these deals, whether it's the 20+ guys the Cubs have already agreed with or the 1st-rounders like Bryant and Gray and Appel, can all go down without a lot of suspense. You get the price in advance, and you don't buy what you aren't able/willing to pay.
Appel had determined that he wouldn't sign for less then elite cash last year; none of the teams who had it considered him worth it, so they didn't pick him. Pittsburgh made a surprising selection, presumably thinking that he'd cut his price, and they were mistaken. I assume the only time things get messy is when a team like Pittsburgh doesn't take a prospect at his word and thinks it will be able to get him for a lot less. Didn't work in that case, and I don't imagine that's a risk teams will often take.