we have wi-fi setup at work using a zone director, and use guest access, with a hidden SSID, WPA-2 personal, and shared password. It is also setup so that the user must click on a terms and conditions page before using the Wi-Fi. This authentication expires after 8 hours.
Most of our devices have no issues, but most of our iPads seem to have this issue where, once they go to sleep, and the user turns the device on again, they have to re-authenticate every time.
Try changing the grace period. Change it to 8 hours or something longer then the default of 30 minutes. The other possible issue is that as the iPad is going to sleep it is sending a deauth to the AP. If it does the AP/ZD will remove the client's entry so it will have to re-auth when it comes back.
Based on your evidence, Sid's thought: " The other possible issue is that as the iPad is going to sleep it is sending a deauth to the AP" may not be accurate. Let us know if iOS 7 solves your problem. We also are in the process upgrading to 7.