For some reason when I recently sync-ed my SplashID between laptop and iPhone it duplicated almost all of my records - this caused me MUCH frustration.

I would like to suggest you add a feature that would warn the user BEFORE a sync. is executed If more than a fixed (or user-defined) amount of data will be changed as a result of the sync. activity.

(iTunes has this feature and defaults to 10%)

then ask the user if they want to continue the sync. operation.