Can't see any better than this at the moment.
Rolling 30 day contract with 500MB per month and access to the regular O2 bonus of BT Openzone WiFi. The dongle costs £19.99 but you can get back £22.50 from Cashback Kings on "all other monthly contracts". Pretty good for those who don't use so much, maybe even for iPad users too? Plus if you don't like it, cancel - no 18 or 24 month contract.