Apple has just unveiled UK data plans for the iPad Wi-Fi + 3G.
For occasional daily browsing it looks like your best option is to sign up with O2. Its £2/day offering of 500MB beats the competition hands down. For the same money you get less than half on Orange (200MB for £2/day) and Vodafone isn't offering a daily scheme.
In terms on monthly tariffs, Orange offers similar tariffs to O2 but also includes a whopping 10GB a month for £25/month. The O2 and Orange monthly tariffs include unlimited browsing at BT Openzone Wi-Fi hotspots. If you want something a little more flexible then Orange has a weekly plan which gives you 1GB for £7.50/week, but you don't get the BT Wi-Fi access included.
Vodafone is offering 250MB/month for £10 and 5GB/month for £25.
We're still not sure if existing customers will be able to upgrade or get discounts on iPad contracts. We're looking into this now.