I got that same error when attempting to use mine. I then found an email in my inbox that said.

"We've received a request to create a new account with [email protected], however an account is already registered with this email address."

So then I used the "Forgot your password?" link and I received an email at my ubuntu.com address to change my password. What is odd is that the link to reset my password was for my primary email address that is listed in launchpad (which is where ubuntu.com forwards to).

So my guess is that when Ubuntu One sees an email address for a ubuntu.com account, it queries launchpad to find the actual email address that it should use. It then links both email addresses to the same account.