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.
Thanks... the OP does not seem to have come back yet.
He said 80% and your's is just five points away, so.... two people's testimony is a bad sign. Something should
change.