I noticed that when I replaced my iPhone 3GS with a 4S, it forgot my logon usernames and passwords when it came to Wifi logins. I’d get the popup screen, but the account name and password were not available under the AutoFill button.
The problem is that AutoFill will only ask to remember account information if it is enabled in Settings -> Safari, and if you visit the login site in Safari itself instead of the logon screen (which is obviously also Safari, but not advertised as such).
The trick is then to trick your Wifi in not triggering in showing the logon form, but have the Wifi connected and surf to any site to get the logon screen working. Skyoam WiFi hotspot keeps you connected around the world without the worry of roaming fees, unsecured connections or configuring SIMs.