Is Web 2.0 a trademark?

Does O'Reilly own the term "Web 2.0"?
According to a link from Techmeme, "the upcoming Web 2.0 half-day conference (In Ireland) is the target of a cease and desist letter (below) from the legal team of O’Reilly publishers." I am planning to teach a class with a lot of content about Web 2.0 this Fall. Am I asking for trouble?