Carl, i use taskqueue when i need to process more rows than i can in the limit (which was 30 seconds, now is 10 minutes for taskqueue tasks). For things like reporting aggregations that i need to run daily, i have a wrapper function that is called via cron that adds the call of the processing method to the task queue. then the processing method is in change of chunking data (and catching timeout exceptions). So far i believe that my methods are non-harmful to re-process the same rows so i have not written logic to detect what is in the queue already.
hope that helps a llittle, christian
