If you break down all the fees for the data and stuff you will see that even if you do "switch" nothing is really changing, so it shouldn't matter.
I'll explain myself for those of you who are curious:
Take the bottom family plan: $129.99 for two iPhones
That's $60 for the main line, $9.99 for the second line and $60 for the unlimited data on each phone ($30 a piece) for 700 minutes
Currently, I have 700 minutes each month and pay $60 for the first line and $9.99 for the additional line. As of right now we have no data.
So, its easy to see that adding the $30 unlimited data plans on top of what I pay now (if I were to get 2 3G iPhones) and switching to the plans announced by AT&T would result in exactly the same price.