I really like Mountain Lion, but the “Check spelling while typing” feature has been tempting me to move back to Linux recently. It’s bad enough that there is no way to turn it off globally. But even more annoying is that it seems to turn itself back on after being disabled in an application.
Anyway, I think I finally found a solution. This seems to disable it for good.
defaults write -g NSAllowContinuousSpellChecking -bool false
Found it here.