The Guardian's environment blog says Paddy Power is taking bets on the world-wide population of polar bears. The key point is that the bet is on the World Wildlife Fund's estimate of polar bears.
So the bet could be rephrased as, "Will a leading global lobby group provide evidence that one of their campaigns is based on inaccurate information?", which may explain why Paddy Power hasn't taken a "take a single bet on the polar bear population increasing."
Now the WWF may be right, but to bet that they will publish data that contradicts their own views is to give them way more credit than I'd be willing to give them. Only a sucker would take that bet.