To keep the balance between the integrity of our trademarks and the ability to to use and promote Ubuntu, we’ve tried to define a reasonable Intellectual Property Policy. You can read the full policy at http://www.canonical.com/intellectual-property-policy. As you can see from our policy, to use the Ubuntu trademarks and and Ubuntu word in a domain name would require approval from Canonical.
Update: From Canonical Blog.
In the case of fixubuntu.com, we were concerned that the use of the trademark implied a connection with and endorsement from the Ubuntu project which didn’t exist. The site owner has already agreed to remove the Ubuntu logo and clarified that there is no connection; from our perspective the situation has been resolved, and we have no issue with the site or the criticism it includes. In fact, far from an trying to silence critics, our trademark policy actually calls out parody and criticism and other uses as being allowed when the marks are used appropriately. (Please make the parodies funny – we need a good laugh as much as anyone!)
From Mark Shuttleworth:
This was a bit silly on our part, sorry. Our trademark guidelines specifically allow satire and critique (‘sucks sites’) and we should at most have asked him to state that his use of the logo was subject to those guidelines.