bombmaniac:
From the ou website:
How is a microwave oven kashered to change the dairy or meat status, or to kasher from non-kosher use? A microwave can be kashered by placing a bowl of water in the oven. The oven is filled with steam by operating the microwave at the highest setting for approximately ten minutes. The bowl is refilled and moved to anotherlocation, and the above procedure is repeated in order to kasher the area where the bowl previously rested. If there is a glass plate on the oven floor, it is preferable to cover or change the plate since it is questionable how the halacha views glass. If the oven surface is plastic there are different opinions whether kashering is effective, but in case of necessity many poskim follow the lenient view. Kashering between meat and dairy can be done immediately after the previous use, while kashering a non-kosher oven requires a 24-hour downtime. In all instances, kashering must be preceded by a thorough cleanup. As is true of a conventional oven, kashering can be bypassed (even for a non-kosher microwave) by double wrapping the food.”