I've received a lot of junk mail after registering

Hello Oskari, I'm sorry to hear that you've been getting spam.

I can assure you that the Ubuntu One servers have not been hacked.

Ubuntu One currently uses https://login.launchpad.net as an OpenID server. This means that Ubuntu One users currently create an account on Launchpad, and then use that to sign into Ubuntu One. We're in the process of changing this login system to something that is more general for Ubuntu, rather than specific to launchpad.

In the last month, Launchpad has been attacked by spammers. While no servers were hacked, they were able to create fake accounts and use them to find some email addresses. When you set up an account on launchpad you get a profile page that lists your email address. This address is not visible to google or to people who have not logged in, but it is visible to other launchpad account holders. What happened is that many fake launchpad accounts were created last month, for the purposes of gathering email addresses. The launchpad team have taken steps to combat this, including identifying and shutting down the fraudulent accounts, and taking even more steps to hide profile pages of people who are not actively using launchpad.

I'm sorry for the inconvenience, and we're continuing to work on steps so that signing up to Ubuntu One doesn't make you more vulnerable to spammers.