>The "z" is from pressing the Shift key to get uppercase letters but accidentally pressing the Z key as well.

That's a questionable theory.
http://raspberryheaven.net/wiki/ZOMG

"The History of ZOMG

ZOMG was originally coined in January 2003 by two individuals, James Curbo and Matthew Milan, over ICQ.

The following chat log may or may not be exact, so apocryphal information has been used to recreate this event.

Hannibal Smith: d00der!
Rick Feynman: zomg!
Hannibal Smith: ZOMG!
Rick Feynman: fag! ZOMG!
"