It will be a "bundle" plan
It will be fierce competition between mobile carriers and phone makers come iPhone's release. iPhone aside, the plan should be cheap enough to attract customers to sign a 2 year plan. That's 2 years of customer "stickiness" for AT&T and possibility of a high customer churn rate for other mobile carriers. The competitors will definitely come out with competitive mobile plans to de-rail the iPhone's success.
No doubt, AT&T will offer a "bundled" plan and hopefully an "unlimited" plan attactive enough for people to switch and buy the iPhone. Without data, the iPhone loses one of its features - Internet Communicator.
Apple wants to "revolutionize" the mobile industry, "unlimited data/text or everything" mobile plans could be one of the items they're trying to "revolutionize" with the iPhone. iPhone or no iPhone, this is great news for consumers. Wishful thinking - maybe.