To a certain extent I am also less carefull when using IM or chats and sometimes I press the "Enter' key before I get to reread what I wrote.
I can't tell you how my girlfriend uses forums though, because I don't think she's a member of any. However, when using IM she tends to keep her eyes on the keyboard and look at the monitor only after pressing Enter.

Now, IM discussions and some forums are often of a more personal nature, so I can understand the less strict self correcting habits.
In some forums however, where one represents the company he/she is working for, I see this "posting without correcting" habit as a rather serious issue. Each post is a mark, an image of the company, and in a way says "this is what we're made of", this is how we answer to our clients' questions, this is how much we know about this subject.
After all, if all other things would be equal, would you prefer a company (hosting company for this board) that writes correctly, or one that makes mistakes when writing in a forum?