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Our connection to iDisk is WebDAV over secure https. It's just like WebDAV in the Advanced sync settings, except that in the case of MobileMe/iDisk, we know what server to talk to and we look up your account name from your MobileMe settings, so we don't have to ask you for those details.

(JKT mentioned the https bit, thanks, but I thought I would make the WebDAV bit explicit.)