With O2 in the UK there is no data roaming for iPad tariff users (not sure about Orange, 3 and Vodafone). You could stick a different SIM card in (e.g. a PAYG one) and in that case it would be £6 per Mb (compared to around £0.09 per Mb via the AT&T card effectively). So yes, still a bargain but not as much as if you did not have to purchase the SIM card for around £11.
Alec
It is wayyyyyy cheaper than the options we have thru ATT for international coverage.
http://www.appleinsider.com/article...data_plans_for_ipad_start_at_25_for_20mb.html