Sam Oliver writes for AppleInsider:
"Already there are a number of threads with dozens of posts about Wi-Fi connection issues, ranging from a weak signal to an inability to connect to a router. Issues have been reported with a variety of routers, including Apple's own AirPort Extreme.You can read the support document titled "iPad: Does not automatically rejoin Wi-Fi networks" on the Apple website.
In response to some issues, Apple has set up a support document for users who have problems getting their iPad to rejoin known Wi-Fi networks after a restart or waking from sleep. The issue is known to occur with some third-party Wi-Fi routers that are dual-band capable."
I've experienced this problem with my iPad yesterday before I knew about this support document. I was able to workaround this issue by powering down my iPad and then rebooting it. I have not had the problem since.