Sorry, its an Urban legend... Snopes.com reports:
FALSE!
Origins:
In August 1996 this story was included in a "Darwin Awards" mailing, marking its first appearance on the Internet. It's a gas, gas, gas! No attribution to a specific news source from which this account might have been harvested from was given, but the date of the unfortunate sleeper's demise was pegged as 25 March 1993. The story has since circulated on numerous "dumb deaths" lists, sometimes with a specious "Bloomberg News" tag attached.
Disabuse yourself of any notion that such a death occurred. Not only don't any bona fide news accounts detailing this bucket-kicking exist, but it would be impossible for anyone to die in such a situation. The gas expelled from the body isn't poisonous; it wouldn't kill anyone. (As for the room's filling up with bodily emissions that somehow "used up all the oxygen" and resulted in the sleeper's dying of suffocation instead of poisoning, the volume of air that would have to be replaced for this scenario to be real is simply way too large.)
In other words, this was all just a made-up story. That it circulates as a "This is true!" shocker shouldn't be taken as proof of anything other than the sender's gullibility.
From a folklorist's point of view, a far better tale is the venerable Fart in the Dark. In that story, the gaseous emissions of the victim don't kill her, though she fervently wishes they had (anything to be spared the embarrassment of the moment!).
Barbara "gas masked" Mikkelson