In Emacs 24.3, I can put the *Messages* and *scratch* buffers side by side,
call eval-last-sexp on message calls in *scratch*, and see the message
automatically appear in *Messages*.

Using Emacs from trunk, *Messages* is not updated with the message until I
either "C-x o" to it or cause some other message to appear (such as "End of
buffer" message).