I think a lot of the people on unlimited are likely not under contracts as the only reason to get contracts would be to get discounted phones, and as I recall to get a discounted phone they would require you to give up the unlimited plan and get on one of the current plans (at least that's basically the equivalent case with Verizon). So I'm not sure that whole get out of contract part (if it would even apply to that, given that there are quite a few restrictions as to where it can apply) would really end up playing that much of a role. PR is somewhat different, but again, hard to say how much of an issue that would ultimately create.
In any case, taking it back to the part of the discussion that started down this path, it's something that can be done, and it's not really something that would clearly make it seem like it would necessarily hurt the carriers, at least in an overall way (or at least to what degree).