Gamasutra reported today that MMO gamers in the US spent $3.8 billion on their hobby in 2009. What's interesting about this figure is that it follows the trend you find in free-to-play games these days: of the 46 million Americans playing MMOs, only 46% of them spent any money at all on games. The rest, approximately 25 million, were completely free-to-play. Even so, American gamers are clearly more enthusiastic about opening their wallets for games than other countries, with the total coming to about 15 times more than any European MMO market.
You can take a look at the full report and more number crunching at Gamasutra.