According to: http://info.astrian.net/jargon/terms/p/pr0n.html

the term "pr0n" originates in kiddie / warez speak. Checking this assertion against a search of Google's Usenet archive gives the following 2 earliest uses of the term:

"Subject: digital fantasies soft porm cd
Date: 1994-06-14 17:33:49 PST
doh!
pr0n i meant!"
And Also dated in 1994:
199.221.129.90 10PM EST thru the night. Days on weekdays. Very Zl0w Pr0n site mail me how to set up incoming access and you guys can upload i have tonz of disk space.

also mail me other zites with pr0n (fsp or ftp) and ill mirror them

All prior references found on Google are uuencoded file attachements.
References:
The Jargon Dictionary
Google usenet search