The p and the o are so close together on a qwerty keyboard that it would be impossible to not get them confused at some point. The possibility of it first being written using a miniature touch screen with a qwerty keyboard is very likely. New cellphones have a guessing ability that relates touch screen area pushes with known words and it is almost impossible to write pwnd from a cell phone now. The lack of an E in the word also leads me to believe that the inventor was careless in his typing skills.
I do not think "pwnd" is a typo. Internet slang is often abbreviated, so that's probably why the vowels are missing. And since it's generally used in a joking manner, I have always thought the 'P' was meant to signify a tongue sticking out, further showing a sense of humor about the usage.
Urban legend has it that pwnd was a typo. It doesn't matter even if it was. It's a unique word and when you say pwnd everyone knows exactly what you're talking about. The typo legend has a certain charm, but it's just a bit too hokey for my taste. I believe it was intentional.