These misspellings seem like laziness for the most part. But I suppose it is also so that people can express themselves in a more unique manner to show some sort of individuality. Personally, it makes more sense to showcase these qualities through what you say and how you express your ideas, rather than forcing your fingers to punch those keys that never seem to get used…

I take more issue with misspellings that grammatically change an idea and often make them confusing. Besides, people look like idiots when they don’t know the difference between your and you’re…but this is my dilemma…

Changes like “dat” and “dis” are probably to cash in on any “cool points” stemming from things that are trendy in modern pop culture.

The typing with numbers part is more than likely related to the earliest days of being online, when various BBS users and people that fancied themselves computer geeks and hackers would use substituted letters and words to bypass content filters. If the word “hack” would get your message banned or deleted, well, perhaps the word “h4ck” or the like would get through. As time went on, typing with numbers took on this connotation of “online eliteness,” especially considering the egos and bravado that accompany alot of of the old-school hackers. (By the way, I use hacker in the neutral form, describing a person who uses technology in order to overcome problems, rather than a person who abuses computers for illicit purposes.)