Should you have a word that you do not want the system to auto-correct (omg is changed to omega, for example), but do not want to permanently add it to the dictionary because it's not a real word, here's a workaround.
In the Language & Text System Preferences panel, click on the Text Substitutions tab, and then put the non-word in both the Replace and With columns. That way, when you type that non-word, it is replaced with itself, and the spelling auto-correction never takes place!

