Cancun climate talks - curbing expectations?

We're told it's the biggest threat to humanity today and that hundreds of millions of people are at risk from its effects. So why have so few world leaders turned up to talks on climate change in Cancun this Tuesday? NGOs and environmentalists are pleading with those that did attend not to miss this opportunity for a global deal, but the will to compromise has been severely hit by the economic crisis and expectations for a breakthrough at this round of talks are low.