I have a iPhone 4 and have ordered the iPad 2 3G
The reason I have is simple. I have an old cut down 3 PAYG sim in my iPhone 4 and when I top up £10 I get 600 texts and 150MB internet for 90 days. I never need to worry about texts, and just top up when I run out of internet - usually after 2 months (depending on usage).
Now to use that with an iPad would mean I have to more regularly topup. I have threfore using an old cut down 3 PAYG mobile broadband sim for £10 gives me 3GB internet for 90 days. I will just top up when I run out like for my iPhone.
Unfortunately 3 have now changed their new PAYG to 30 days from 90 days, but have left it for legacy users!