[piperswe]

They do keep track of when you leave by commercial air or sea, and I think there are data sharing partnerships with Canada/Mexico as well. Not sure if they keep track for citizens though, since there's no immigration enforcement reason for that.

[technothrasher]

They're working on an "entry data is automatically sent to other country as exit data" system with Canada. I'm not sure they've got any decent partnership with Mexico currently, though I know they've been negotiating.