Hi Anders,

Does the attached patch work for you?  It's still a bit rough around the
edges, but seems to do the right thing.

A quick test on 10.6:

(setq ns-auto-hide-menu-bar t)

;; Places the frame just above the top of the screen.
(set-frame-position (selected-frame) 0 -20)

;; Frame would be completely offscreen; so it's constrained and brought
;; back.
(set-frame-position (selected-frame) 0 -10000)

Cheers,
Charles