Been covered a million billion times already, not applicable to existing tariffs. O2 have confirmed this...
"New and upgrading customers will continue to receive unlimited data as a promotion until 1 October. From that date, they will be able to buy additional data Bolt Ons which include 500MB for £5 or 1GB for £10 and O2 will ensure customers have the ability to easily monitor their use by sending them regular text messages.
All O2 customers will continue to receive unlimited Wi-Fi through The Cloud and BT Openzone hotspots.
Existing customers will continue under their present terms, unlimited data usage with an Excessive Use Policy.
Ronan Dunne concluded By doing this, we are laying the foundation for a sustainable data experience for all customers and the huge possibilities that technology will create over the coming years.