Remember that sweet deal that came with the iPad 3G where you could buy unlimited access to AT&T’s (T) cellular network for $29.99 a month with no two-year contract required?

Well it’s going away — 40 days after it started — as part of the sweeping changes in its pricing plans AT&T is putting into effect on June 7. In its place are the $15 DataPlus and $25 DataPro plans described below.

It’s all part of AT&T’s effort to relieve stress on its beleaguered network by going after the heaviest users — the 2% of its customers who push or pull through it more than 2 gigabytes per month.

Some of those customers, however, will be grandfathered in. If you are an existing AT&T customer with an Apple (AAPL) iPad or an iPhone — and there are some 20 million of you — you have the option of sticking with the $29.99 all-you-can-drink plan. At least for now.

Below: The details of the new AT&T pricing schemes — including a new $20/month iPhone tethering plan that will become available when iPhone OS 4.0 arrives.

From AT&T’s press release:

• 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 tethering-enabled 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.

[Follow Philip Elmer-DeWitt on Twitter @philiped]