You missed a recent alley cat in Madison -- When Pigs Fly.
As was already mentioned bikelikemad.org is a good site to keep an eye on that stuff.
There is an alley cat here in Milwaukee on the first Friday of every month. There is also an annual messenger invitational.
If you want to hook up with other Madison riders, there are rides Wednesday evenings (At six I think) at the top of State St. by the capitol.