Not a lot, seems there is an exchange called the European Climate Exchange claims to be associated with the IPE in london? It would seem to be an easily manipulated market but you never know.
Why do you think it could be easily manipulated? http://www.europeanclimateexchange.com/index_flash.php http://www.chicagoclimatex.com/about/
Just wondering how do you prove that you are not polluting more or less than you say you are. If enough people are fudging the numbers I would imagine supply/demand will go out of wack and prices won't reflect reality. I suppose thats not manipulation of the actual credit prices though.
From what I can see, it's a really clever scheme to cap pollution. I've only done some cursory reading though. Also, if set up right, the price should, in theory at least, reflect the average cost of reducing pollution. If you can buy polluting plants with lots of credits, take cheap steps to bring down the pollution and sell the excess credits you have a good business. And environmentalists should love you. Fixer upper power plants!