I had a friend in college who confidently explained that cigarette butts biodegrade and that it was OK to toss 'em on the ground. We argued a lot.
Cigarette butts are not biodegradable. In fact, they're just about as far from being biodegradable as you can get and are made of synthetic fibers that act as a sponge, sucking up and holding a plethora of toxic chemicals from the tobacco smoke.
When cigarette butts get tossed into lakes, streams and other waterways, they can wreak havoc on the ecosystem, killing off fish and other aquatic life.
Jennifer Lance over at Blue Living Ideas has a great post laying out the dangers of discarded cigarette butts and pulled together some pretty scary numbers: 1.69 billion pounds of butts are improperly disposed of (i.e.: littered) every year. Swing over and get the full (scary) picture.
And if you smoke cigarettes, stop tossing your butts in my environment.