Last Across the Finish Line
"Last Across the Finish Line" is an example application that uses the Task Queue API to showcase a pattern for tracking asynchronous batch workers and notifying a user when all the workers have completed.
This was described in detail in a three part blog post from one of the Google App Engine team members.
The pipeline API is a higher level way to orchestrate task queues (this has been merged since in the mapreduce API we ship with the SDK).
There is also a third-party library -- fantasm -- for doing similar task queue orchestration.