According to Boy Genius Report, as of June 7th, AT&T will not be serving up unlimited data for its smartphone customers. It looks like they are really pushing for users to go with Wi-Fi as an option to bogging down their 3G network as they include in the press release that

Each plan includes unlimited access at no additional charge to more than 20,000 AT&T Wi-Fi Hot Spots in the U.S. Customers can also use unlimited Wi-Fi at home, in the office or elsewhere if available.

These are the new plans and although current customers will not be required to change they can choose to switch to a new plan without a contact extension (in fact, I think AT&T would be happy to switch you over!):

• DataPlus. Provides 200 megabytes (MB) of data – for example, enough to send/receive 1,000 emails (no attachments), plus send/receive 150 emails with attachments, plus view 400 Web pages, plus post 50 photos on social media sites, plus watch 20 minutes of streaming video – for just $15 per month.**  This plan, which can save customers up to 50 percent off their wireless data charges, is designed for people who primarily like to surf the web, send email and use social networking apps. If customers exceed 200 MB in a monthly billing cycle, they will receive an additional 200 MB of data usage for $15 for use in the cycle.  Currently, 65 percent of AT&T smartphone customers use less than 200 MB of data per month on average.
• DataPro. Provides 2 gigabytes (GB) of data – for example, enough to send/receive 10,000 emails (no attachments), plus send/receive 1,500 emails with attachments, plus view 4,000 Web pages, plus post 500 photos to social media sites, plus watch 200 minutes of streaming video – for $25 per month.**  Should a customer exceed 2 GB during a billing cycle, they will receive an additional 1 GB of data for $10 for use in the cycle.  Currently, 98 percent of AT&T smartphone customers use less than 2 GB of data a month on average.
• Tethering. Smartphone customers – including iPhone customers – who choose the DataPro plan have the option to add tethering for an additional $20 per month.  Tethering lets customers use their smartphones as a modem to provide a broadband connection for laptop computers, netbooks or other computing devices. Tethering for iPhones will be available when Apple releases iPhone OS 4 this summer.

As far as iPad users go, AT&T said:

For new iPad customers, the $25 per month 2 GB plan will replace the existing $29.99 unlimited plan. iPad customers will continue to pre-pay for their wireless data plan and no contract is required. Existing iPad customers who have the $29.99 per month unlimited plan can keep that plan or switch to the new $25 per month plan with 2 GB of data.
(via AT&T via Boy Genius Report)