For those who doubt that the WMM issue can prevent an iPad from connecting to a particular router I provide the following from the Apple support website. Apparently the problem is principally with 802.11n networks if WMM is not supported:
Symptoms
Apple products may be unable to connect to 802.11n Wi-Fi networks if WMM (Wireless Multimedia) is disabled.
Resolution
This applies only to third-party routers. Apple AirPort products do not have a setting to enable or disable WMM.
WMM must be enabled for devices to connect. Disabling WMM on your wireless router can prevent the following Apple products from connecting to an 802.11n Wi-Fi network:
iPad
iPhone
iPod touch
Apple TV
If you are unable to connect to an 802.11n Wi-Fi network on any of these devices, check to ensure that WMM is enabled and configured correctly on your wireless router. If you require additional assistance, please contact your router manufacturer.
If you would like to read the full item on the website it is here:
WMM support