hikerchick - not to be paranoid, but I would do some more research if you are serious about using the wood. I don't know a lot about pressure-treated lumber, but I remember about 15-20 years ago when my parents put in a deck, my mom wouldn't let me eat a sandwich if I dropped it on the deck! I think especially older pressure-treated is more likely to have really sketchy stuff in it (I have heard that they don't put arsenic in it any more). I hate to be a fear-monger without any real information for you, but when it comes to dangerous chemicals I would never assume, "Oh, it's probably fine."
That said, reusing materials is awesome and I hope you find evidence to convince yourself that it's safe to use the stuff. I'd be curious to know what you find out. Good luck building your coop either way!