In case you missed it early this morning, AT&T's new mobile data plans (DataPlus, DataPro and Tethering) have given up on the idea of unlimited data in favor of a split tier of 200MB at the low end ($15) and 2GB at the high end ($25), with a $20 supplement for tethering (but hey -- tethering! and only six months late!). There is also a new high-end plan for the iPad, $25 for 2GB as well. In the midst of the ire and anxiety, a few points have been clarified over the past few hours.
If you buy a new iPhone, you are not obligated to accept the new plan terms; existing plans will be grandfathered, according to a post on AT&T's Facebook page. iPad owners can also grandfather in an unlimited plan, although it's not clear what happens if you drop month-to-month coverage.
As pointed out on MacRumors, you can use AT&T's account management tools to check on your own data usage and see which of the new plans is closer to your run rate. You can also just dial *DATA# from your iPhone.
AT&T also reminds users that the company's Wi-Fi coverage is free to all customers with an existing data plan.