I'll bet someone (mythbusters or similar) has done a check of the old "dog in the microwave" urban legend (presumably using, one can hope, melons or sausages or something as stand-ins). My suspicion is that if you put a melon in a microwave, it would burst in one area or crack open as the pressure inside rose, but that it would not be vaporized/evenly destroyed. So you should be left with large parts of the, um, melon, intact or grossly fractured, so something could be hidden inside it at burial. (Microwave science refresher course: microwaves work by essentially causing water molecules to vibrate at higher frequencies*, thus creating heat. Inside a closed rigid space like a skull or a melon, you would expect to get an expanding steam problem like a faulty pressure cooker, leading to a blowout along whatever weak point or fault line might exist. How's that for Monday morning coffee reading?)
*Oh, and has anyone else ever done the trick where you put a CD inside a microwave? NOT SUGGESTING YOU DO THIS, but it was kinda popular to do during the era when AOL was sending their damn intro disks in the mail all the time: You get these cool circular races of sparks as the disk warps -- I've always assumed DFW had some image of this in mind if J.O.I. believes he is destroying a cartridge in his head, due to the odd annular patterns created. Been too long since I learned how microwaves worked to remember if there is anything particularly "annular" about the energy pattern of the vibrating water molecules.
ETA: Ok, here's the best video I could find on the basic effects:
http://www.youtube.com/watch?v=VRtVcYAc ... re=related(It's a lava lamp being microwaved...and I'm never searching for "melon microwave" again, because I learned about something I wish I hadn't!)