I don't have the specific documentation that says this is the case, but I personally canceled my 3G iPad preorder and bought a WiFi model yesterday at the advice of an Apple Store employee.
Just call your local Apple Store and they will verify it for you.
Update: You guys scared me, so I just called Apple directly. They said that the official policy is that you have 14 days to exchange an opened Apple product, but in the case of the iPad it is common for Apple Store employees to extend this to 30 days (I now see the 30 day extension note on my receipt).
I did not get the phone call in writing though, but as I mentioned, you can verify it for yourself. ;)
Go ahead and call your local Apple store if you don't believe me. :)