Are you sure it is actually wobbling or does it just LOOK like it's wobbling. It is kinda hard to explain, but the plastic rotating shroud seems to be cast off-centre on many vehicles, and it looks like it is wobbling when it's not.
As long as it is running true and the edge is not wobbling, then it may be just the way it looks. (It is very strange and I scratch my head every time I look under the hood. Maybe someone on here can explain this or describe this better.)
Follow Roadie's advice and pull back and forth on it while stopped to see if it is loose on the bolts or if the water pump bearings are going bad. If it's solid, take a real close look at it and see if it truly is wobbling or if the casting is just warped or off-center. (As long as it stays in balance, it should be okay.)