Cool idea - I have the same problem and once started tracking a "last contact date" in my OS X address book. Then I had an AppleScript that would go through and check that date and give me 3,6,9 month reminders.
I'd love something that would connect with Twitter and automatically identify when I say "we should meet up" with somebody and remind me to meet up. Because I nearly always forget when I say that, especially if the person is in another city.