When to cancel AT&T Data Plan?

I'm considering buying one of AT&T PDA phones from ebay, they are free there. The question is: will there be a problem if I cancel the data plan before 6 months? I heard that some people were able to cancel it with AT&T without having to pay any penalties.

Whether there is a fee depends upon the underlying reseller. AT&T itself does not sell phones on eBay. It sells either through its online website or through its own corporate stores. Any other sales are through independent dealers. You should be informed of the conditions of the sales and activation before you purchase the phone.