"Y'all" is correct. The apostrophe is being used to replace the "ou" in "you." Can y'all tell I've been studying for a grammar, spelling, and punctuation test for my major? Just had a study session today, and my professor harped on apostrophes for a good 15 minutes.
BTW...thanks Corey and Will for getting the Podcast up so quickly!

Off to listen to it now!