It's pronounced "four-twenty"

a coworker found this on theantidrug.com and passed it along. i think it's pretty funny. maybe you will too...

Drug Lingo: Know What to Listen for

"Have you ever heard your teenager reference the time “4:20?” Many parents don’t realize that 420 (pronounced "four-twenty") is a “secret code” for a time to get high. The reference to 420 presumably dates back to '70s stoner lingo but is still widely recognized by the youth of today. Some people have even designated April 20th as "National Pot Smokers Day."

If you hear your teenager reference 420, see that he is using the term while instant messaging with friends or has a 420 sticker on his car or backpack, call him on it. Let him know you know what he's talking about and set up a time for a longer conversation about your family’s no tolerance policy for drug and alcohol use."