Not quiet, the use can have tasks running, like trackers (e-Mule, torrents, etc), or an installation of some big program that takes a lot of time, or any other long task that requires respective CPU usage. Which means that i need to get from the system in some way information that no key was pressed and the mouse wasn't moved for a "suitable" amount of time.
Like when ICQ drops to Away mode after a while that the PC wasn't touched.