Irule that waits?

As Colin said, tmm execution (and therefore iRule execution) is single-threaded. This was done for a variety of reasons, not the least of which is that you can be guaranteed that nothing else will happen while an iRule event is executing, so users don't have to worry about (e.g.) mutex locking on global variable accesses. No other connection can stomp on your global data in the middle of you accessing it. CMP introduces some complexities into this; likely someone will have to write an article on this sometime soon.