It was April of 1988 when Delta Airlines banned smoking on its domestic flights with a duration of two hours or less. The rest of the airlines followed suit. Soon the industry had extended those flights to less than six hours or less in 1990, and all domestic and international flights by the year 2000. Interestingly enough, pilots were allowed to continue to smoke on the flight deck after the 1990 ban due to concerns of nicotine withdrawal in chronic smokers until 1995. Don't try to disable the smoke detector in the lavatory either, should you choose to break the law you will not only be placed under arrest upon landing, but can be subject to fines of up to $3,300 in most states.

More Info: en.wikipedia.org