It's been a little over four months since the initial data plans for the iPad were announced, and slightly more than a month since the 3G iPad's been available. So, why the rapid about-face? It probably has something to do with the more than 2 million iPads sold to date, and the iPhone's increasing global share. This is most likely overwhelming an already-stressed data network, even though AT&T insisted that they could handle it.
Currently unlimited data plans for existing iPhone and iPad customers are still in effect. However, once contract renewals come up, don't be surprised if you have to wave bye-bye to unlimited data. The press release did not say if the $20 tethering option would be available to those who decide to keep their unlimited data for now, but MacRumors believes tethering won't be available unless you're on the 2 GB "DataPro" plan. Unfortunately, they're probably right. Update: They're right. Tethering is only available as a buy-up from the DataPro plan, and is not an option for grandfathered unlimited-plan users.
The new data plans take effect June 7.