If you want certain models, you're going to have to wait. The Apple Store at Chandler Fashion Center only had the 64GB Wi-Fi model while the Biltmore Apple Store had 16GB and 64GB Wi-Fi models. Since I only wanted the 16GB model, I walked away a happy customer. But, if you want a 32GB Wi-Fi or any of the 3G models, you're pretty much out of luck.
If in doubt, though, call ahead if you can. If you make a spur-of-the-moment decision and your store is out, see if the Apple Store you're at can call another location and check on their stock. Of course, this method only works when you live in an area that's lucky enough to have more than one Apple Store. Independent resellers or Best Buy stores are also worth a try.