the last 3 signings I went to were at a tower books and 2 at a barnes and noble. They were very strict about getting other things signed. you had to buy the book there at their store (no copies from home) and that entitled you to get ONE signature on THAT book - i think it really depends on who is signing and if it's through an agent or directly with the author (Athlete)
Bring something else to get signed just in case, but don't get your hopes up -
on a related note, if anyone goes and has any idea how much it will cost for the Josh Hamilton book and some travel/time expenses, let me know and I would be interested in buying one, if the price is right