Operator detection?

> I've been wondering this for a while... screen savers have 
> some way of 
> detecting activity on the keyboard, mouse etc.  I'd like to 
> expose this
> info for other purposes; like for instance maybe you could 
> finger me to 
> find out if I'm actually sitting in front of the computer right then.
> And instant messaging clients could use the info to go into 
> "away" mode
> after some timeout period, like they do in Windows.  Any idea 
> how to do
> this?
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