Your spouse asks you a question and you answer ... then wait for an up vote.

When you're answering this question.

Your boss has to post on one of your questions just to get your attention.

When your business partner puts the following on the company's white board

(stackoverflow is not work)

When you find yourself learning about XYZ just to get one more answer in.

When you've written 4 lines of code all day.

You start trying to downvote emails

You keep pressing the refresh button hoping that there will be fresh questions in your chosen tag

You're on another website and wonder where the controls are to vote content up or down.

You've run out of votes ? :)

Someone you haven't met refers to you as "Sara from Stackoverflow"

When you check StackOverflow first thing in morning - even before your official email! :)

You type an 's' in your address bar and it auto-completes to 'stackoverflow.com'.

When the F5 has been worn off my keyboard ;-)

When the top domain in your web browser history is sof.modos.org. (if you don't know yet...)

When your name is on the first page of the user list.

Jon Skeet emails you to suggest walking away for a bit.

when you can easily tag new duplicate questions in a second (with a link to the original or similar question)..

When you know a search on StackOverflow with Google should be done with:

site:stackoverflow.com -hottest -highest -recently -newest

Other criteria:

When you know the ASCII code for ( and ) characters, because javadoc html links can refer to parameters API within rounded brackets...

http://myPage/myObject.html#myFunction(myParams)

will have to be written

http://myPage/myObject.html#myFunction%28myParams%29

When you see a question and think:
"Great! I really would like to know the answer to that one".

Only to discover that it was you who had answered it.
And that it has been the most upvoted.
And your answer had actually been accepted... sigh.

When you are talking to a co-worker and you keep looking for the button to close as "Not programming related".

When you complicate your architecture at work just to have to ask interesting questions on stackoverflow.com

When you remember the day the Have you become addicted to Crack Overflow?-Question was asked :)

Your friends get envious of your reputation climb rate

When you prefix all of your responses to actual people with "+1" or "-1".

When you are on Stack Overflow while at PDC2008 instead of attending sessions . . .

A recursive function you wrote fails with a "stack overflow," and you start looking for the vote up button in the function declaration.

You go out of your way to post an answer on this thread rather than getting on with the mountain of work that you have to do...

You've broken 10,000 rep points.

Or, you know who the first person to break 10k was.

You have a twisted notion of what "building a reputation" is.

When you have tabs open to all of your active questions and all of them have ReloadEvery enabled.