Not sure how much of a surprise this will be to most people, but if you take a look at the iPhone FAQ section on Apple’s UK site, you’ll notice that it says that the iPhone 4 micro-SIM can’t be used in the iPad. V3.co.uk reports that Nick Wilkins, press relations manager at O2, told them that if you place an iPhone 4 micro-SIM in an iPad, the SIM will not work. So, anyone hoping to switch micro-SIMs between the two and share data allowance, for example, will be disappointed. If you just want to revert your micro-SIM to your old phone that uses a standard SIM, according to V3.co.uk, O2 has said that this will be possible using their supplied adaptor. Source: V3.co.uk
That's probably on that network only. People have successfully cut their regular sim cards to micro size and used them in the iPad.