Hi Dmitriy,
I had the original prepay plan that was only offered through iTunes (I think it was only offered if you failed the credit check process). I bought the iPhone from my local AT&T store but did not set up service at the store. I went home and set up the account and payment method through iTunes. The unlimited data package was $15/month.
This was in July 2007 but I'm not sure how long the plan was offered. Until last month, I had never had a problem with the plan or the service.
Here is the link AT&T sent me when I was notified of the coming change:
Thanks for the help,
Kyle