Alex Chitu at the unofficial Google Operating System blog reports that:
Gmail’s code reveals an upcoming feature called “magic inbox” or “icebox inbox”, which is likely to prioritize the messages sent by your friends and other contacts you email frequently.
That wouldn’t be hard to implement for Google or any other email service / application that has access to your history, but I’m skeptical of the value of implementing prioritization this way. I can’t speak for others, but I personally have no reason to believe there is a correlation between frequency of contact and priority. Indeed, I’ve found that non-spam out-of-the-blue emails are sometimes the most pressing ones, e.g., requests to write something for a publication or present at a conference. Not to say that my more frequent correspondents aren’t important, but if anything they have other ways to reach me with time-sensitive requests.
I’ve pushed for attention bond mechanisms before, and I’ll do it again. I’d love to see them implemented in a way that plays well with the infrastructure and is usable. To my knowledge, they are the most promising way both to improve spam filtering (though, in fairness, current spam filters work adequately) and to prioritize non-spam. But I recognize that the infrastructure and usability hurdles are significant.