iPad Won't Log Onto Wifi - Safari Bug
This problem is a known bug in Safari on the iPad. Select SETTINGS and then SAFARI.
Turn OFF the autofill function until the problem is fixed.
You might find that it works when you connect at a normal WEP network, then you go to work or an airport where you have an OPEN Wifi, and it won't work. Or it will work one day at the airport/open network but after you use the iPad at the WEP network and go back to the airport or open network suddenly it won't work again.
Just turn off the function and you will be fine. It will be fixed in iOS4 and in any future Safari for iPad that comes out.
Hope this helps.