I bought my ipad2 in January and bought an ipad data sim with 3gb loaded on it at the same time- it cost about £19. Ive since found them for around £12 on a well known auction site. Mine are on 3 who seem to be the popular provider in the UK. The sim card doesn't expire, but the data usage does- the 3gb expires after 3 months, the 1gb after 1 month. You can top up directly from the ipad when on 3g - just switch the wifi off and connect to the provider. You cannot top up if connected by wifi. 3 has a mobile app and you can connect free to check your balance and to top up, even when you have nil balance left. It costs £10 for 1gb access for a further 30 days. However, if you want to save money you can buy a few data preloaded sims from the well known auction site and simply change them as your data expires - much cheaper to pay £12 for 3gb than £10 for 1gb! The sims remain valid even when the data expires though, so you can just top them up if its inconvenient or too fiddly to change them and put new ones in.