I think this will complicate things, the carriers always want to control access. AT&T was the only true punch and play in the iPad. Tmo always wants to charge for sims, Verizon - forget it. Plus I get special deals, special promos on tmo. For more complicated users I think it will not so easy.
Remember the wonderful stop and go unlimited at&t plan for the magical iPad1. That was used to sell iPad and AT&T stopped it month in, (I did just get a $45 check from the lawsuit over that)..
I don't trust the carriers but if this takes some of their over control away, ok.