Toronto is sending one, and the White Sox are sending one. OSF linked to a Farmstros blog post speculating about specific players - mainly guys who were drafted last year, and signed less than one year ago. But it seems to be anybody's guess, really.
I know there was a Luhnow quote about the PTBNL for the Myers deal being a "significant piece" or some such, and I'm thinking/hoping he will be the best "prospect" in that deal, since, despite the Astros' chipping in a good hunk of cash, the two known guys don't appear to be much to write home about. The Toronto guy I'm guessing won't be much, but based on the seeming rabbits Luhnow was able to pull out of his hat in the Lee, Happ, and Johnson deals, I think we have reason for optimism on the Sox PTBNL.