Wikipedia deems this topic significant enough to merit its own article:
Fart lighting, or pyroflatulence, is the practice of igniting the gases produced by human flatulence, often producing a flame of a blue hue. The fact that flatus is flammable, and the actual combustion of it through this practice, gives rise to much humourous derivation. Other colors of flame such as orange and yellow are possible with the color dependent on the mixture of gases formed in the colon.
Although there is little scientific discourse on the combustive properties of flatus, there are many anecdotal accounts of flatus ignition and the activity has increasingly found its way into popular culture with references in comic routines, movies, and television; including cartoons.
Unfortunately, its sources, which include Answers.com and YouTube, leave much to be desired. For starters, YouTube videos can easily be faked.
Is there any evidence that flatus is flammable? If so, are there any verified cases of its ignition?