I am very much in the same boat as i have a rolling 30 day simplicity 25 contract with unlimited data, i too have decided to purchase direct from Apple a couple of Sim Free iphone 4's... now i know there is a lot of interest & speculation in whether the standard sim can really be modified to work in a iphone 4, there are a host of how to do guides, but are they really backwards compatible.
Apple have stressed that a iphone sim will NOT work in a ipad, however people who have modified standard sims appear to be able to use the data facility on a ipad without any problems.
I would be very interested to know people thoughts on this issue, and how would i go about ordering a new micro sim.