If you're going to get an iPad 3G you'll need to a 3G provider. iPads are sold network unlocked and require a micro SIM, a miniature version of a normal SIM card. Though it's possible to bodge a micro SIM with a little elbow grease (see Google), the official line is you'll need to sign up with one of four providers (O2, 3, Orange and Vodafone) that are offering iPad tariffs. Luckily SIMs are provided for free and contracts are short, so if you're not satisfied with one supplier it's easy to switch.
Orange has two rolling 30-day contract tariffs, one daily rate tariff, one weekly rate and offers a pay as go option as well.
iPad plan: This is Orange's pay as you go plan, which charges for data at 5p per MB. It's a nice option to have if you're just quickly checking something, but the potential for racking up a bill is quite high. Orange does apply a £40 limit, though.
iPad Daily: For £2.00 you get a 200MB limit until midnight that day. Enough for a whole day out, though unlike some other tariffs access to Wi-Fi hotspots isn't included.
iPad weekly Similar to iPad daily, but this time you get seven days access with a 1GB limit for £7.50 - around £1 per day effectively.
iPad Monthly 15: A 30-day rolling tariff that costs £15 per month. It has a 3GB limit and features unlimited access to BT Openzone Wi-Fi.
iPad Monthly 25: For the power-user only, this tariff nets you a 10GB limit and unlimited Wi-Fi but at a cost of £25 per month.
Orange offers the most choice of all the providers and has Wi-Fi included on its rolling contract tariffs, but the cheapest of those is £15 per month. Weekly and daily options are nice to have, but the weekly option looks poor value in comparison to the £15 contract and you don't even get a full 24-hours access using the daily rate.
O2 has three options, all of which include unlimited access to both BT Openzone and The Cloud Wi-Fi hotspots.
500MB per day: Like the Orange daily rate this costs £2.00, but you get more 3G data and Wi-Fi access as well.
1GB per month: Though the limit is quite low, it costs just £10 a month and you get excellent Wi-Fi access. If you reach your limit you can top-up in increments of 500MB.
3GB per month: Aside from the addition of The Cloud's Wi-Fi hotspots, this plan is identical to Orange's
On the whole O2's plans offer better value than Orange's. Its daily rate is particularly good, offering more data and Wi-Fi access for the same price. Its 1GB per month plan is a good option for regular but light use.