You wanted to know if the person set their status or if the computer is idle and Messenger set it. Well as you can see by the screenshot two people are idling, meaning they haven't touched their computer in x many minutes and Messenger detected it and set their status to idle (or away with an unmodified exe)
Like i said in my previous post, if your contact sets their status Messenger will not override it with idle. So when the status is actually idle (like in my screenshot) means they are away cause there is no user activity on the computer. Does that make sense at all?