80% of bitcoin trade happens at Mtgox which is centralised.
How do you get 80%? Is there a well-defined set of bitcoin addresses that belong to MtGox so that we can compute the ratio of the value of transactions involving them to the value of all transactions?
You can work out roughly what percentage of bitcoin trading is happening through Mt.Gox by looking at the volumes of the different exchanges listed at
www.bitcoincharts.com/markets. In the last 30 days, roughly 2.1 M bitcoins have been traded through Mt.Gox, and roughly 2.8 M have been traded through all of the exchanges combined. So Mt.Gox has about 75% of the market.