Coming soon to GMail: See who is accessing your mail

This is absolutely brilliant. It's been done on the web before but rarely and usually not so well. Of course, it's the same thing you get on logging into a shell account and using "w" or "last" but doing it on the web is really a big deal. Every web site with access control should provide some sort of log like this. It seems like a simple enough idea but why isn't everyone doing it?