Google Analytics is already in for the public facing stuff (there have been 46 unique visitors in the last month). There's no reason why we couldn't add it to the admin pages as well.
(Use firstname.lastname@example.org to log into GA)
We can already show the last login time (as Craig has said). It's not impossible to add the extra column to see when their last request was as well - I'm not sure if last login includes users authenticated using a "remember me" cookie.
I'll add the last login details to a report when I get the axslx gem installed... hopefully soon.