Alberto, I understand that our implementation uses a POST mechanism for redirecting queries that requires Javascript enabled. That may be disabled or not supported on mobile devices.

Chris Frazee posted on this a while ago..I have attached his comments

I incorrectly attributed this to Chris Frazee...Kevin M is the real author Please be aware that iOS enabled devices (a.k.a iPhone, iPad, iPod) do not operate in the same way as OSX, Linux and Windows devices do with Captive Portals. Most operating systems require the user to manually launch a web-browser once they connect to a Captive Portal Wireless LAN for the web-browser session to be captured and redirected to the Captive Portal. This is a common process and is something that most end-users are familiar with.
When an iOS device is connected to a network with a Captive Portal enabled, the iOS device will launch a sperate application Apple calls a 'Web Sheet' that presents the login page to the user. This application IS NOT the Safari Web Browser but rather a stripped down web-browser that has limited capabilities. As this is not Safari can't tell you if it supports proxies or not! The 'Web Sheet' will launch if the iOS device detects that a Captive Portal is present on the network and will launch as soon as a iOS device connects to the Wireless LAN. The 'Web Sheet' will also close as soon as the user authenticates or accepts the terms and conditions and iOS device is on the network. As the iOS device uses the 'Web Sheet' and not Safari, any re-directs that are performed after the user authenticates or accepts the terms and conditions (i.e re-direct to a welcome page) will not be performed by the iOS device. This is because the ‘Web Sheet’ is a separate application to Safari which closes as soon as the user is granted access to the Internet. This is expected behavior and is something that Apple has specifically implemented on the iOS devices. There is currently no way that we (or Apple) know of that will allow Safari to be launched automatically from the 'Web Sheet' after authentication has occurred. We have asked Apple this question but have not received any response or work around. So the with this in mind once the iOS device has connected and authenticated to a Captive Portal using the 'Web Sheet', the user will have to manually launch Safari to browse the Internet. This is exactly how Apple expects the devices to behave and this is regardless of any vendors Captive Portal solution that has been deployed.