Radix, there is not currently such a feature.  That would be possibly more complex than it sounds like.  You'd have to determine what you consider idle to be.  Clients often send lots of packets on minor presence changes (idle, etc), there's whitespace pings that keep the connection alive, etc.  If you were so inclined, you could write a plugin for Openfire to perform this task.  It would have to pick what it wanted to watch for and keep it's own registry of sorts of who's idle or not idle.