That will be great news, I went to AT&T yesterday and got a Micro SIM card (unactivated and not tied to my iP3GS number) that they have for the iPad and I was going to use that as a back up. But from the reserach I have done on that particular Micro SIM it probably will not work in the new iP4's.
Great news guys, I was successful in tying my existing line to the iPad Micro SIM card to my iP3GS. Both voice and data works with this SIM card.
For those that are not aware the AT&T iPad Micro SIM has two cut outs the first cut out is the same size as the regular SIM that all AT&T deivces use now (except for the iPad and the iP4) and another cut out for the Micro SIM.
When I receive my iP4 I will use the second cut out and will not have to worry if the one that comes with it is activated to the other line I upgraded from.