I haven't really investigated until now, but I believe this problem has been with me since installing Tiger: for some reason it only shows thesaurus entries for me, not dictionary entries. I unchecked the dictionary in the preferences, and now it doesn't show up in the list AT ALL (just the thesaurus). The "New Oxford American Dictionary.dict" file is there in /Library/Dictionaries. Any idea what's going on? I'm at a loss.