The Wall Street Journal has a good article about leetspeak, the shorthand text many have adopted for communicating in the digital age (think "LOL" for "laughing out loud").
Leetspeak first became popular in the 1980s among hackers, or "elite" users who developed the coded language to conceal their hacking plans and elude text filters. It has since become a common method of communicating - via instant message, cell phones, and email. Teenagers [allegedly] like the fact that their parents can't read it; adults often find it a useful shorthand when communicating on the fly.
While language purists are outraged at the abandonment of proper grammar, some scholars argue that it's natural for language to evolve in this way. The articles cites examples such as the word "OK" (first appeared in a Boston newspaper in 1839 as an abbreviation for "oll korrect" and is now one of the most commonly used words) and "SOS" (the popular call for help, originated during the telegraph era when people communicated via dots and dashes and abbreviations). The Coyote Blog has a great post (via MarketPower) likening English to an Open Source Language, meaning it evolves based on how ordinary people are using it.