Timestamps on IM status. e.g., your friends can see that you set your status to “Out to lunch” 5 minutes ago, and so are not likely to be available in the next hour; or that you set it an hour ago, so you’ll probably be back soon.
Status is one of the things that makes IM so useful: it’s a passive way to keep in touch with friends and coworkers. A timestamp would make it even better.