(package-built-in-p 'thingatpt '(0 0 0)) ; emacs 24.4
nil
(package-built-in-p 'thingatpt '(0 0 0)) ; emacs 24.3
t
but:
(package-built-in-p 'thingatpt) ; emacs 24.4
(thingatpt . [nil nil "get the `thing' at point"])
(package-built-in-p 'thingatpt) ; emacs 24.3
t
If you check list-packages you will also not find thingatpt in the list of builtins in 24.4 which cannot be correct, right? Only packages that have a version are listed which is the minority.