If you look on Vodafones eForum you might find conflicting advice on whether the company can unlock 3G iPads bought from them on either PAYG or on a contract. The answer is a definite yes, as I have managed to get mine unlocked with relative ease.
Don’t bother phoning customer services they wont be much help. Instead, head over to this web form. If your account is Pay as you go, then apparently you’ll need to top up £20 to cover the service (but don’t quote me on that, I’m not on PAYG. Try phoning customer services… I guess).
Type in your iPads phone number (which should be on your contract paperwork, if it isn’t like mine you can try another number on the same account (which worked fine for me!) otherwise again you might have to try calling customer services)
Complete the form, wait about 3 working days (you can leave your computer and go about your life during this time) and you should get an email back from Vodafone telling you its been completed. Enjoy swapping sim cards in your freshly unlocked iPad.