For those of you planning to just switch out your iPad microSIM in to your iPhone 4 when it arrives next week, Apple confirmed that doing this will not actually work...
In the FAQ section on the iPhone 4 page the question and answer are provided as follows...
Q: Are iPhone 4 and iPad micro-SIM cards interchangeable?
A: No. Your iPad micro-SIM card will not work with your iPhone.
While it says the microSIMs from an iPad are incompatible, we believe that this is not so much a compatibility issue on a technical side, but rather the functions limited of the microSIM from an iPad which does not have voice enabled on it. When calling the phone # from an iPad you just get an error saying unable to connect.
However, a spokesperson for O2, one of Apple's partners for the iPhone and iPad in the UK, stated the devices are "quite different."
"Both use a mini-SIM, but the iPad micro-SIM is not, for example, provisioned for voice, and focuses on flexible data. The micro-SIM for the iPhone is set up for voice and other functionality."
A hack will likely be developed for this!