Final counter 3: Block IPs not trusted; all central blocks would then go into thin air - ONE update.
torDoesn't matter: If I see a 51% attack and set my client to only trust the mtgox IP for instance then you can fake coming from all the IPs you want using TOR, but that will never get your block on my client.
It's funny I am called the pig here when other posters are displaying such immense ignorance of basic TOR/IP/internet function.