Gone is the $30/month unlimited data plan. On June 7, there will be a $15 200 MB/month data plan called "Data Plus" and a $25/month 2 GB data plan called "Data Pro". And AT&T finally got off their lazy butts and added a tethering option( which includes the iPhone in OS 4.0) for an additional $20/month and requires you to get the Data Pro plan.
http://www.macrumors.com/2010/06/02/...ne-data-plans/
Current iPhone customers can keep the $30/month unlimited plan if they desire. They can also keep it when they upgrade to a new iPhone. But, you won't be able to get tethering.
http://www.macrumors.com/2010/06/02/...r-next-iphone/
For me personally, I will take the $25 2 GB Data Pro plan and get the $5 savings. I hardly use 200 MB( but do get too close for comfort to do the $15 plan) so 2 GB limit will be fine.