Not for the first time, I found myself at a hotel last weekend at which neither my iPhone nor iPad would connect to their wifi, while my MacBook Pro connected just fine. (Why did I have all that gear? I had a reason, not relevant here. Trust me.)
The details: both iDevices were running iOS 4.3.5. The hotel was a Best Western, and the network login page mentioned colubris.com. Colubris is in the network management business, and was acquired a while back by HP. When I’d try to connect, a login page would appear, and when I entered the username and password that worked on my MBP, I got a blank page in return, with no relevant recourse but a Cancel button.
(I’m told that this kind of authentication goes by the term captive portal.)
Anyway. If you find yourself in this situation and really must connect, here’s what worked for me. Go to your devices’s Settings app’s Wi-Fi page. Find the network you’re trying to connect to (in my case it was named SpeedLinks), and tap the blue detail disclosure button. There, along with some other stuff, you’ll see an Auto-Login switch. Turn it off.
Now connect again, and use Safari to browse to some website. You’ll be presented with the login page (which you may have to zoom bigger in order to complete), and this time the login should work.
Update (August 2012): iOS seems to be getting better at this kind of thing. With the current version (5.1.1), I haven’t seen this problem, even at sites that used to cause trouble (though I haven’t been back to the offending Best Western yet).