Scrivener is a popular writing application for the Macintosh.
Auteureist™ has built-in support for exporting via email to Scrivener's MultiMarkdown format. See the section on Exporting via Email To Sccrivener.
Currently, the only way to go in the other direction (get text from Scrivener into Auteureist™ is to use the device's built-in Cut, Copy and Paste support. See the section on Exchanging Data via MobileMe, and Importing Text.
For more information on Scrivener visit their site: http://www.literatureandlatte.com