The following article is technically talking about the work/office environment, but the lesson applies just the same to the online message board and blog environments.
A choice quote:
Misspelling can be interpreted as: “Boy, he/she is stupid.” An occasional spelling mistake is not going to label you stupid. But if you make spelling mistakes in everything you write, then people may believe that you were at the bottom of your class.
If it looks like a duck, swims like a duck and quacks like a duck, then it probably is a duck. You can easily tell who the stupid people are.