One of the worst case global warming scenarios is the release of frozen methane trapped under the arctic. That is happening now.
Also that 20 times more damaging than CO2 is actually if the methane is released over a 100 year period, the ocean melt is expected to be over a much shorter time. Dumping the methane into the atmosphere over a 20 year period makes the methane 62 times as effective at trapping heat as CO2.
Methane Clathrate and Climate Change
Methane, It's a gas, is a Nature blog post arguing that maybe the media reports are perhaps to inflammatory since we don't know how long the ocean has been bubbling up over in Siberia, so I'm going to back down from my previous paranoia.