One additinoal note...if a team drafts a player in the first three rounds and he doesn't sign, they get an additional pick the following year in roughly the same draft position as the original pick.
However, in that second year, if the player they draft with that pick doesn't sign, they get no compensation pick. In other words, that pick is only protected in the first year.
For example, the Diamondbacks, Padres, and Brewers all get a compensation pick this year for failing to sign players in last years first round (Loux, Whitson, and Covey, respectively). Whoever they draft with those picks this year will have to sign or they won't get any compensation next year. So you better believe the players these teams picks in those specific spots will be guys they know will sign.