For those interested in the topic of emissions trading, a new piece was just published by the International Emissions Trading Association on the topic of 'linking' different emissions trading regimes (PDF document).
Linking entails allowing emission credits from one scheme to be rendered tradable in another. For example, European credits would be valid and tradable in California, and vice-versa. Beyond allowing the carbon market to become more efficient and liquid, linking could also present a range of arbitrage opportunities.
For all of you environmental markets fiends out there, I would definitely recommend this paper. It's short (13 pages) and gives a good overview of where things are currently at with emissions trading and the possibilities associated with linking.