Preview at WWDC likely in June, followed by September launch.
AT&T Replaces iPad Unlimited Data Plan with 2GB Capped Plan for New Customers
At present, AT&T offers a no-contract $29.99/month unlimited data plan specifically for iPad owners. If you activate your data plan before June 7th, you can still participate in this plan. After June 7th, however, AT&T will be replacing that plan with a $25/month 2GB limited plan. From the press release:
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.So, if this is an issue for you, you still have time to be grandfathered into the unlimited data plan. Existing iPad customers can look at their current data usage via their iPad settings.