According to Apple [AAPL], the Micro SIM cards for the iPhone 4 and Apple iPad are not compatible, meaning you wont be able to use your iPhone 4 Micro SIM in your iPad and vice versa.
This seems a bit of a shame as you will have to have two separate data plans for your iPhone 4 and iPad, you can see below what Apple says about it.
Are iPhone 4 and iPad micro-SIM cards interchangeable?
No. Your iPad micro-SIM card will not work with your iPhone.
I guess that clears that up then, although we are not sure why you couldn’t use your iPhone 4 SIM card in your iPad just for data, is there something that Apple does to your iPhone 4 SIM card when you activate it through iTunes that stops this.
According to Vodafone, who told one Geeky Gadgets reader, the Micro SIM card for the iPhone 4 and Apple iPad are exactly the same, and if you order an iPad SIM card now, you will be able to activate it through iTunes as an iPhone 4 SIM card.
Make sure you check out our guide to getting a Micro SIM for the iPhone 4 in the UK, which we will update as soon as more information is available.