Sorry to post in such an old thread but I have to say with my limited knowledge I would hesitate quite strongly to heat anything flammable in a microwave. If you have spent a considerable amount of money on the microwave you MIGHT be safer but as far as I know now microwave heats even remotely evenly. Just say you're microwaving a burrito (at least it is a relatively safe bet) if you stick the burrito towards the center of the microwave you'll usually notice (at least in cheaper models) that the endpoints of the burrito are MUCH cooler than the center. Even given that for PG this is a (relatively) safe method (assuming there is no metal in there and that the vapor will never reach the radiation source which if I'm not mistaken does spark) if you're microwave is failing usually this leads to even greater temperature gradients which may or may not cause it to get above the autoignition temperature (this is of course a college student approaching this from the microwave burrito perspective... Your mileage may vary).