Problem 1: My brain thinks much too much faster than my fingers can respond.
Problem 2: My fingers type words on autopilot - as in, they're used to certain combinations of letters, and so brain says word, fingers type word, there no longer any spelling involved. Well, not consciously anyways.
Problem 3: I don't bother going back and spellchecking.
Solution? Slow down, I guess. But that's hard. ::sulks:: Or I could start spelling everything that I type.