[whpalmer4]

Open up System Preferences, .Mac or MobileMe, click on the iDisk tab, set the synchronization to Manual instead of Automatic. As I recall, you'll get a little circular arrow dohickey next to the iDisk which if clicked will cause a sync to happen. To the best of my recollection, you can't write anywhere on the iDisk volume that wouldn't be synced.

I still think having another program (MobileMe iDisk sync) trying to sync your precious OmniFocus data file between your machine and the MobileMe server without following the locking protocols used by OmniFocus is placing a lot of faith in the notion that Murphy's Law doesn't apply to you :) My inclination, if I had to have an offline copy of my iDisk, would be to make my own disk image and use a third-party sync utility that could be configured to ignore the OmniFocus data file.