While there is ample evidence that diverting attention to spelling when writing "disrupts the planning process" of writing, there is an alternative to those who propose spending more time on direct spelling instruction:
Advise writers to delay focusing on correct spelling until their ideas are firmly in place, while, at the same time, building up spelling competence through massive reading.
A number of studies show that good writers delay editing concerns until the final draft, and "premature editing" has been shown to be a predictor of the frequency of writing blocks.
Sounds about right to me. At the same time there are strategies that we can teach students to self correct and at our school we are looking to have David Hornsby come to work with us on a whole school approach to teaching these strategies.
Krashen's final piece of advice: "let spelling develop naturally through massive reading in the early years, and provide older writers with some guidance in the use of spell-checkers and spelling dictionaries, as well as advising them to delay spelling concerns until the final draft."
I think this is something the movers and shakers of the Aussie group, the Literacy Educators Coalition have been saying as well.