I am betting that it was just your run-of-the-mill airplane crash. Here's another one that seemingly "disappeared off the face of the earth": Stendec. That one even spawned a UFO magazine. It's real fate, although more mundane, was still interesting; the aircraft buried itself in the top of a glacier and was then carried to the bottom by the moving ice, where it emerged 50 years later. No aliens, no conspiracy, no suicide, no intrigue. I imagine that the remains of Mr. Fossett or his plane will be located eventually as well.