I've run into a bug with GNU Emacs 24.0.50.1 (i686-pc-linux-gnu, GTK+
Version 2.16.1, checked out from the repo a week or so ago). Let-binding
frame-local variables doesn't work: although the let-bound value is correct
within the `let', it doesn't get reset after the `let'. To reproduce, just
run http://gist.github.com/403208.