It’s BBCode, and is used on many other message boards. I’m sure some folks were using it before this place was a twinkle in Cecil’s eye. 
When you get warm fuzzies from a whole thread full of folks being just as freaky as you. 
…when you get irked at another poster on another bb and type, “I Pit you!” and then get 10 replies asking wtf you meant…
When your wife asks “Anything interesting on the Dope today?” before she asks “How was school?”
When you say “Cite?” to your students.
When a student asks you a question you don’t know the complete answer to, but you are sure that Bricker or John Mace or Sam Stone or lissener or Qadgop or…will know more than the kid ever wants to know on the subject.
When you say in conversation “A buddy of mine in Afghanistan…” and you remember that 1) you’ve never met Tripler anywhere IRL, and 2) that you still consider him a buddy.
When your work surfaces (whether computer desktop or coffeetable or office desk) are littered with text notes for potential thread ideas being subjected to careful contemplation and development.
(The currently successful moments-of-absent-mindedness thread is the result of a week of consideration and refinement. Yay! On the other hand, the predict-the-next-collectible thread I started a couple of years ago, after a similar period of fine-tuning, died like a roach in a deep-fryer. Boo!)
When one of my employees does something worthy of me blurting out, “Whiskey Tango Foxtrot!”
I think if we work at it long enough, that one will pass into general usage.
So many of my conversations are started with, “The other day on this message board I’m a member of…” or “I read the other day that…” and expecting other people to get as jazzed about stuff I read here as I do.
When you get pissed off at something in the news, and realize you’re not going to be able to let go of the anger until you pit the idiots involved.
Or the reverse–some celebrity makes an ass of himself, and rather than getting pissed, you think, “I can’t wait to see the Pit thread that this spawns!”
When you have to start claiming that you “heard it at work” because your wife is tired of hearing about the SDMB twenty times a day.
While someone’s speaking to you, you’re looking over his left shoulder, checking his join date, post total and location.
The Straight Dope logo is burned upside-down into a corner of your retina, purple letters on a yellow background.
When you have an orgasm, you yell, “Oh Og!”
Hearing of an event somewhere in the world and thinking “I wonder if we have someone there?” and then reading first hand of the volcano in Alaska, the hurricane in the Gulf, the shooting in Florida or a cyclone in Australia.
…when you make a goof like this. :o
I have no excuse. I’ve seen the notation “HTML code is off” on this message board for years. Maybe I’ll go eat worms.
I’ve done it on another board… and…actually said it to someone - and I’m a newbie.
When your SO becomes a member of the Dope and you spend 20 minutes debriefing her on the overall rules of the board and the rules for the individual forums.
I felt like Lee Ermey screaming at Vincent DiNofrio in Full Metal Jacket.
However, she would’ve left me if I made her walk around, pants around ankles with her thumb in her mouth.
Especially if you sound like Seinfeld saying “Newman!” 
When you say to someone, “Well, I’m not sure, but I know this (prison doctor / soldier in Afghanistan / expert on the postmodern aspects of Paul Verhoeven’s films) who might know…”
When your co-worker, at the bookstore, shows you the copy of Angelina Ballerina she just bought for her niece and you are compelled to direct her to the relevant thread.
When making plans to travel out of town, your immediate thought is, “Hmm. I wonder if [vacation spot] will be having a Doper gettogether then. If not, maybe I should hold out until they do.” Yeah, that’s me!
Nah – a real Doper would announce his or her iminent visit in MPSIMs and start a dopefest from scratch!