I thought mine was pretty good, but heard a better one in the lobby of the hotel where I'm staying this morning. Mine was that a government shot down the plane (either intentionally or by accident), the U.S. know who and where, but we want to keep it a secret for some reason so we're using this time to clean up the wreckage so it will never be found.

The new one I heard is that the plane was essentially "stolen" and didn't actually crash. It's on a tarmac somewhere being loaded up with explosives (and/or nuclear weapons) by some terrorist group or foreign government. At some point in the future they're going to fly it to a destination of their choosing and detonate.