Early adopters often encounter technical issues with their gadgets and it seems that Apple's iPad is no different as many sites and blogs have been documenting users complaining the iPad's 'widespread' wifi problems (remember the iPhone's wifi issues?), and even on Apple's own forums, owners are raising the issue (here and here).
“Under certain conditions, iPad may not automatically rejoin a known Wi-Fi network after restart or waking from sleep. This can occur with some third-party Wi-Fi routers that are dual-band capable when: Using the same network name for each network, [or] Using different security settings for each network,” (source)
Gadget Lab goes on to note that: "If all else fails, Apple recommends resetting network settings in the iPad’s Settings app (Settings > General > Reset > Reset Network Settings) and also suggests: For a deep dive on the different types of Wi-Fi bands, see Glenn Fleishman’s article “Understanding Wi-Fi’s two spectrum bands.”