Indeed. My money is on "bomb" just because it's hard to come up with a maintenance problem that causes such a rapid disaster at altitude. It would have to be something that caused a massive explosion like TWA 800. Possible, but unlikely. The lack of a claim makes a bombing less likely too, but IMO less so than the improbability of a mechanical problem.
As I understand it, you'd expect such a thing to happen during the climb. That's what happened with your linked flight 611, if I'm reading between the lines correctly, and it's what happened on e.g. Aloha 243. In general it would make sense that the failure would typically make itself known as the stress on the fuselage is increasing, not after it's reached a steady state. Not that it's impossible, but the fact that this flight was at cruising altitude for a while before it disappeared would seem to be an argument against this possibility.
