My username is unrelated to the 'deck of cards' one could think of.
When created my first email account in 1995, I knew of the roleplaying game called Shadowrun, in that world people that used the 'Matrix' were called deckers because they used 'cyberdecks', kind of keyboard+modems connected to their brains...
So as I was becoming a internet user, in a sense I was becoming a 'decker', I could have gone for 'netrunner' as in the other game called Cyberpunk 2020, but I guess I found 'decker' cooler at the time !
http://www.boronk.de/SHADOWRUN/Decker.jpg