in OS X system preferences you can set sticky modifiers. I used it for a while with emacs, but finally abandoned it because, while easy on fingers, it tends to cause strange things if you start typing a combination and halfway you change your mind (you can just release a modifier key usually, but if you just typed it there is nothing you can do to undo it, except C-G).
When you wrote that, did you know that if the sticky-keys setting is set and you accidentally hit a modifier key, then you can undo that by hitting the key 2 more times?
(I've been using OS X's sticky-keys setting with Emacs for years, BTW.)
Yes, I recall having noticed that, but still didn't feel very natural, probably because I use different systems (not all of them OS X even) and this complicates habit making.
