>So when the user does something... *something* happens. Even if it is a "system busy" response. Anything is better than nothing happening.
Actually if I remember correctly what happens is that the OS will silently drop UI requests from the user. The OS maintains the queue from the UI to the application thread and if it gets built up it will just start dropping items from the queue. So that if you click 'send to printer' but the application is busy it may or may not completely ignore your request.
This is one of those things that the OS did to make things 'simple' for the application developer to write threaded applications. I could be very mistaken, of course.