This is all conjecture. I haven't seen anything that claims this on any news/blog/rumor site anywhere except people making weird claims on the forums.
You need to search a little harder. There are a lot of news sites reporting the pricing. Click
HERE for one of them.
Under the section titled
"Motorola ATRIX™ 4G" you find this sentence in the third paragraph
"The first combines ATRIX 4G and the Motorola Laptop Dock for a promotional price of $499.99 after a two-year service contract and $100 mail-in-rebate after subscription to Data Pro smartphone data plan and tethering add on.". This indicates that to get the package deal you have to sign up for the Data Pro plan which is $25 and the tethering add on which is an additional $20.
If you go down to the quotes section under #3 you see this
"Laptop Dock - Firefox browser use with AT&T Mobile Broadband requires Tethering Plan.
". #4 of the same section is indicating that tethered devices require the DataPro Tethering Plan (which at&t could also call using the HD dock tethering).
So all in all, if you're buying the phone by itself, you don't need to change your data plan if you currently have a smartphone data plan. If you are buying one of the docks, be cautious, as they will more than likely require you to change it.
Now here is my speculation on why at&t stated this pricing to news groups and not on their website. They are hoping that the pricing of the atrix will convince a large majority of the people that were excited about it to change their mind and purchase the inspire 4g that comes out next week. They will then release a statement on their website with "revised pricing". Again, my speculation.