Its what is called an "Easter Egg" Just something the programmers hid in the software when they created it. Once you have posted 123456 posts it will show as "I am a geek"
You cant change it other then changing the post count. When you post again it will go back to normal.
Other Easter Eggs:
Your post count will show as "leet" if you hit 1337 posts
here is one that SMF for Free added:
Login as admin on your forum and change the action in the url to ?action=maintain;sa=destroy in your browsers address bar