Although I was initially skeptical, today's events and the joint statement have me convinced that MtGox is indeed insolvent.
However, is it actually possible for that 750,000 BTC figure to be true? You cannot lose that much to a software and accounting problem "without noticing for two years!" I see three possibilities:
1. The private keys to the cold storage addresses were accidentally destroyed, lost or seized.
2. The coins were stolen by insiders using "malleability" as an excuse.
3. The 750,000 BTC figure is wrong, and the number of missing coins is much less.
This figure comes from a "crisis document" which is so badly written even Gox couldn't have made it! (clearly fake)
This fact doesn't clarify the real situation unfortunately.