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.