My wifi has been working well at all times in my house, where I have an airport base station, and in several other locations with wifi.
But today I visited my mums house and after signing in to the wireless, the iPad didn't see it. I turned wifi on the iPad on and off and no more problems for the rest of the day.
I put it down to the fact that the router is crap! As it's a cheap one I bought just for when i visit. I know that's not really much help to the OP, but I'm convinced it was due to the poor router I had an issue.