Have you been getting age verification requests wanting you to ring a number or register a credit card to prove your age :? Seems a bit dodgy ... just like the sites I am trying to get on when it pops up ...
Bango are a billing and analytics company that O2 are using to verify that you are old enough to use 'adult' sites (gambling, "other....").
I see the reasons for all this but you need to be over 18 to get a phone contract so why this sudden change? It's been handled extremely badly as well; no advance warning, very poor presentation of page, wrong font etc.
If you use wi-fi however, it bypasses it completely but with 3G you're stuck with it I'm afraid.