How Likely Is a Runaway Greenhouse Effect on Earth?

If "climate engineering" fails and the planet gets warm enough to melt the polar ice and heat the sea beds to release the trapped methane, its probably inevitable!

("Methane is one of the most potent and influential greenhouse gases on Earth. The 100-year global warming potential of methane is 25, i.e. over a 100-year period, it traps 25 times more heat per mass unit than carbon dioxide." http://en.wikipedia.org/wiki/Atmospheric_methane)