It's probably worth pointing out that you'll have to cut down a regular SIM card to make it fit the iPad if you go for one of the regular deals, and not everyone will want to do that. And a monthly contract won't suit all users either; I hadn't planned on getting a 3G-enabled iPad because I didn't want to commit to another contract, but the 5p/Mb 'pay for what you use' tariff from Orange changed my mind - and my pre-order
Pete