A similar technique seems to be in use by at least one other UK network.
I'm with 3 (one of O2's rivals) and if I access my account information from my handset connected to their network, it takes me straight to the relevant info without a pesky login. (If I try the same via wi-fi, I get a page asking me to disconnect from the hotspot and go back online via 3G.)
I always did wonder how they achieved this--sending the mobile number in the HTTP headers is likely.
It's possible that they set the header at their gateway so that it will override whatever you set from your device. It's also possible that the app is firewalled to only be accessible directly from their mobile network.
Of course, it's also possible that at least one operator isn't following appropriate measures to make this secure.
I'm with 3 (one of O2's rivals) and if I access my account information from my handset connected to their network, it takes me straight to the relevant info without a pesky login. (If I try the same via wi-fi, I get a page asking me to disconnect from the hotspot and go back online via 3G.)
I always did wonder how they achieved this--sending the mobile number in the HTTP headers is likely.