IM, Posting, e-mail: how do you (should you) write?

Crazy question in a world without rules: Is there a big diference between the way you write in the following: • e-mail (informal; not business related) • instant messaging • posting I'm talking about grammar, spelling, punctuation, conventions (not content). Do you write differently on all three? is there a real etiquette different (again...non-content) between the three or does anything really go?>