I recently ran into a problem on Mac Safari 5.1 where I couldn't type into any text boxes in my Flex App. Very frustrating! Anyway, I finally found a simple solution from the Adobe Forums.
First this problem only happens with the Debug Flash Player.
The fix is very simple - in Safari under the Develop menu (if you don't see the Develop menu it can be shown by going to Preferences > Advanced and checking the Show Develop menu in menu bar checkbox), change the User-Agent to Firefox or something other than the default. This will cause the page to reload, and should allow you to type again.