In the IRC channel - johto suggested an implementation:
<johto> if you want to get really fancy you could have two columns where only one of set; one would be the value (if reasonably unique) and the other the id (if not)
I only suggested how to do this in an application if one really wants the feature so badly. I don't think this would fly as a native implementation.
I'd like to add that an ENUM can be used instead of the id+lookup table in the 2nd column for non unique values.
