I've always been told that you aren't supposed to put drain cleaner in a garbage disposal, so I haven't done that. I saw a suggestion on metafilter for using baking soda, with or without vinegar... I may try that. The problem with using a plunger is that I couldn't find a good way to block the air gap.
I've poured a bunch of boiling water through, as well as quite a lot of hot tap water. I've also tried cleaning the garbage disposal with ice. Oh, and poured quite a lot of Dawn through it.