i've thought of something along these lines (latest version of quicktime required to view)
http://www.esopenko.com/video/aquarium/sprinkler.mov
Just need to figure out how to attach the sprinkler portion to the piping, and allow movement. I think you can control the speed at which it's spinning through the friction/tightness it's attached.
I think these steps should work:
- cutting a groove along the top of the "inner" pipe
- the outer pipe is cut in half length wise, then a groove cut in each piece so it fits into the inner pipe
- glue the split pipe back together over top of the inner pipe