ok from what i understand about sim cards (and i might be wrong here but this is what I understand from reading varius sites), is that it is easy to delete messages, modify teh address book, etc. (all phone features). However, to do anything else, i.e - add your own new data it gets very complicated and it involves unlocking the phone etc.
If you are looking for identification using phones why not use bluetooth, almost all phones come with this feature now. Plus the user doesn't havent to take the sim card out of his phone all the time.
Also not all phone companies use sim cards. Bell and Telus in Canada, which account for approx 50% of cell phone users, here use CDMA and most phones don't have sim cards