At&T and Apple came up with 3 plans for iPhone. the cheapest will be $59.99 per month and will include unlimited data (web and email).
iPhone goes on sale at 6:00 p.m. (local time) on Friday, June 29 and will be sold in the US through Apple’s retail and online stores and AT&T retail stores. iPhone will be available in a 4GB model for $499 (US) and an 8GB model for $599 (US), and will work with either a PC or Mac®.
All iPhone monthly service plans are available for individuals and families and are based on a new two-year service agreement with AT&T. Individual plans are priced at $59.99 for 450 minutes, $79.99 for 900 minutes and $99.99 for 1,350 minutes. All plans include unlimited data (email and web), Visual Voicemail, 200 SMS text messages, roll over minutes and unlimited mobile-to-mobile and a one-time activation fee of $36. Family plans are also available.
and here's a good review of the iPhone.