Spammers actually find this corruption of an already corrupt term useful, as it allows their junk e-mail to bypass weak automated mail filters, such as those employed by America Online. This is one of the reasons spam usually has so many "typos" in it (besides the fact that most spammers have IQ less than their shoe size...)

To me pr0n refers to any digital pornography, whereas porn refers to pornography in dead tree format. Also, its not really 1337 speak, as it seems most of the geek/nerd/hacker/whatever crowd uses it.

PPN = P = precedence lossage

pr0n //

[Usenet, IRC] Pornography. Originally this referred only to Internet porn but since then it has expanded to refer to just about any kind. The term comes from the warez kiddies' tendency to replace letters with numbers. At some point on IRC someone mistyped, swapped the middle two letters, and the name stuck, then propagated over into mainstream hacker usage. New versions of the Mozilla web browser internally refer to the image library as "libpr0n". Compare filk, grilf, hing and newsfroup.

--The Jargon File version 4.3.1, ed. ESR, autonoded by rescdsk.

Most importanly, I think, this now-common Internet convention was adopted in order to circumvent (or simply to mock) Net Nanny-style blocking software, like those set up by "concerned parents", public schools and universities.

When these software packages originally became popular, webmasters feared that any mention of the word "porn" on their pages would get picked up by automated search routines, and some adopted this alternate spelling just to keep their sites off the ban-list.

As it turns out, this is probably a bit of an overreaction, since the algorithms used by these applications are usually more sophisticated than a simple one-word text search. Still, the habit persisted and soon became second-nature.

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

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