This is a classic case of trademark confusion. The question is what the best course forward is, and I'm not 100% sure I see a way out in which both end users and the Arduino TM holder would all benefit. If he sues then a good chunk of the money that was deposited by KS backers of the project will be spent on lawyers, if he doesn't he's making it clear that the Arduino trademark is not defended which will lead to a lot of copy-cats.<p>Tricky situation.