Despite the largest protest since...help us out Paul Soglin...there are no reports of violence. From what I saw, it is unlikely that serious rioting would arise from the crowd at the Capitol. It if does, it will likely come from protestors who are unaffiliated with the unions.
When I ran into Zach Winiewski, from Blogging Blue, he told me that the several thousand union members who congregated at the Masonic Temple before the protest were warned to resist provocations from Tea Party activists, and told that violence would only threaten their cause.
I don't know if the Club for Growth buses ever got into town. One Twitter commenter put it this way: "Seems the only counter-protesting at the WI state capital today: people who think main protest is not liberal enough."
However, if they did, it's doubtful that their presence was even registered. There were so many protestors, many of them carrying American flags, a staple of the Tea Party movement, that it would be hard for a counter-protestor to make his presence known.