If you haven't moved your keyboard or mouse for a duration of 5 minutes, then you would start logging idle time. This is an issue of the job at hand, because jobs like your job (and writing/designing/thinking) don't require the worker to always be on the computer.

We considered periodic webcam screenshots, but we find that to be a bit creepy and overbearing. oDesk does that, but I'm not sure thats something we want to do.