Note that while we don't attempt to restrict uses of the "duino" suffix, its use causes the Italians on the team to cringe (apparently it sounds terrible); you might want to avoid it. (It's also trademarked by a Hungarian company.)
As far as I can tell, they tend to focus more enforcement effort on companies who sell sub-par ripoffs and defects as genuine. It looks like stamping them out is a full time job.
It is covered here: http://arduino.cc/en/Main/FAQ
Clones and fakes: http://blog.arduino.cc/2013/07/10/send-in-the-clones/
As far as I can tell, they tend to focus more enforcement effort on companies who sell sub-par ripoffs and defects as genuine. It looks like stamping them out is a full time job.