I don't think a truly drop-proof case is practical for the iPad. It would need tons of shock absorption to be something you can count on for, say, a drop from five feet onto asphalt. It could be inches thick in places. Increasing the dimensions of a gadget (particularly a glass screen) increases it vulnerability geometrically. What works for an iPhone probably won't work for an iPad.
Then again, I'm not a mechanical engineer, and I might be overly pessimistic. Lots of cases give at least some protection all around the iPad, including mine, the InCase Convertible Book Cover.