The closest I could come up with was to create a set of queues (that mimic threads). These queues hold items of work that need to be carried out and once every 250ms or a developer configured amount, the queue will ask a worker object to find out what work needs to be carried out. If there is work to do then it will start the task, if work is already being executed from the queue, then the work will not initiate the task.
Check out my AJAX Tagger 2.0 and see psuedo threads in action.
If someone else has any ideas about threading in Javscript I would love to hear it.