Nobody's officially getting Panther before the 24th--so you'll have to do the $19 thing.
I'd wait until AFTER the 24th, and then make sure you aren't getting a "last week's" model. It should then have Panther in the box. Maybe not pre-installed yet, but the install is easy.
I ordered my eMac the day Jaguar came out, and Apple said I'd have to send away for Jaguar. But lo and behold, it WAS in the box!