IMO, spellchecking should be disabled into notes because one often type here some custom stuff and abbreviations such as "IMO", "BTW", ..., "smth", "manuf.", etc., which aren't to be parsed by the spellchecker. IMHO, notes aren't intended to be spelled correctly. It is annoying to get useless spellchecking stops into notes every time.
IMO at least a "Disable notes spellchecking" checkbox into the spelling dialog (or the preference pane) would be useful. Enhancement? Mael.