Symptoms:
Under certain conditions, iPad may not automatically rejoin a known WiFi network after restart or waking from sleep. This can occur with some third-party WiFi routers that are dual-band capable when:
* Using the same network name for each network
* Using different security settings for each network
More Info:
http://support.apple.com/kb/TS3304
Note: Always ensure that your WiFi router firmware is up to date.