Hey Apple! Leave my SIMs alone! :)
I travel a lot for work - and when I travel, I often have a local SIM that I can simply pop into my phone without having to worry about excessive data roaming charges. For voice, I usually forward my mobile number to my Vonage line which can then "simulring" my foreign SIM, thus saving me vast amounts of money.
SIM cards are great! They are small enough to be easily transportable (I have about 10 in my wallet at the moment) and large enough to be easy to handle. I'll never have an iPhone 4 for a variety of reasons (I did buy my wife one though), but one of which is that it is almost impossible to get hold of prepaid microSIMs in the various countries I regularly visit.
So, I see no benefit to the microSIM as is, and a smaller one even less so. As to dumping the SIM entirely, and letting someone like Apple control my network via something like iTunes - absolutely no way!