What's interesting is someone on another forum said this foil might actually make it WORSE. The way a microwave door is designed is that there is a very tiny gap, it is specific to block that wave length. If you block that gap with something metalic the RF might actually couple with it and cause leakage. That foil could basically be acting as a (rather inefficient) antenna.
Though considering just HOW inefficient of an antenna it is, the VSWR would be so bad it may as well be a shield. So it's probably fine.
RF is basically voodoo magic.