Google has now rolled out a new match choice for keywords.
Where previously the choices were exact, phrase and broad match, they have now added a modified broad match option.

To implement the modifier, just put a plus symbol (+) directly in front of one or more words in a broad match keyword. Each word preceded by a + has to appear in your potential customer’s search exactly or as a close variant. Close variants include misspellings, singular/plural forms, abbreviations and acronyms, and stemmings (like “floor” and “flooring”). Synonyms (like “quick” and “fast”) and related searches (like “flowers” and “tulips”) aren’t considered close variants.

I think this will find wide use in fine tuning the fairly common situation where negative keywords aren’t useful for rejecting unwanted searches, but exact or phrase matches are really narrower than you want to go.