(defn stamped [vctr f & args]
  (conj vctr (apply f (assoc (peek vctr) :stamp (java.util.Date.)) args)))

user=> (def v (atom []))
#<Var@3af7345b: #<Atom@61128f5a: []>>
-----
user=> (swap! v stamped assoc :name "Joe")
[{:name "Joe", :stamp #<Date Mon Feb 14 12:19:23 EST 2011>}]
-----
user=> (swap! v stamped assoc :name "Bob")
[{:name "Joe", :stamp #<Date Mon Feb 14 12:19:23 EST 2011>}
 {:name "Bob", :stamp #<Date Mon Feb 14 12:19:28 EST 2011>}]
-----
user=> (swap! v stamped assoc :last_name "Smith")
[{:name "Joe", :stamp #<Date Mon Feb 14 12:19:23 EST 2011>}
 {:name "Bob", :stamp #<Date Mon Feb 14 12:19:28 EST 2011>}
 {:last_name "Smith",
  :name "Bob",
  :stamp #<Date Mon Feb 14 12:19:41 EST 2011>}]