can we get a way to get to the time stamp for role creation?

when i create a role like "bob.is_member_of ruby_meetup" i'd like to present on the membership list page the date on which bob became a member like "bob has been a member since 8/22/2009".  i couldn't find a simple way of accessing that time stamp.  i'd be more than happy to fork and add it myself if you could suggest the logical place for it.  thanks.
