I've seen this in message but have no idea what 420 is?  Any help?

Mary Jane? One of my ATF's... (eom)

There was a thread related to it on one of the boards here awhile ago but i can't find it at the moment.

-- Modified on 8/25/2003 5:32:20 PM

and i'm not exactly ignorant of drug lingo ...  what's the exact reference to? ... 420 what/who/why?

Apparently 420 --> "Time to smoke marijuana"
See link for history, using an article from High Times mag as reference (or is that reeference?) -- seems "420" was the time of day when a group of stoners would meet daily to get high.
As the article says, "We did discover we could talk about getting high in front of our parents without them knowing by using the phrase 420."

and NO ... i'm not affiliated with Google Inc.

I thought 420 was LE's code for a drug arrest.  I don't know.  We need someone from "TMR" the Marijuana Review to help us out.

it fell on Easter this year.

I've heard the explanation of 4:20 PM too. But I tend to believe the national holiday explanation as the original reason.  As time has gone on, the time of day has been accepted by enough people that has merit too.

4/20 is usually earth day.
Most take it to mean 4:20pm on 4/20 is the day everyone who smokes pot is smoking.  Bonding sort of thing.

Since I am familiar with 420 I will fill you in.  In the early 90's some stoners (mostly Deadheads and the west coast kids) got the idea that a 420 was police lingo for "person smoking pot".  To the best of my knowledge it's not in any state.  Anyway they all started smoking at that time of day, a kind of symbolic toast with all the other shady smokers.  
The added bonus is parents and coworkers have no clue what it means.
Peace out, make love not war!

Also it could mean that it is the international dialling code for Prague. LOL