Strange, but very aggravating issue. I use a blackberry, which is with a BES on my work computer... Now, everytime I sync in between Omnifocus and iCal (at home), the sync does something to my task records... not sure what... but after I sync OF and iCal, i then use Missing Sync for Blackberry and all of my completed tasks in iCal, are suddenly incomplete on my Blackberry!... though the very same tasks are still marked complete in both iCal and Omnifocus. Then (and this is the maddening part) when I sync again with my blackberry, all of my completed tasks in iCal become incomplete again!

It does not matter whether I mark the task complete on my blackberry, in iCal or in Omnifocus... the same issue occurs. My preferences have "add completed items to calendars" checked in Omni-focus and I sync completed tasks in missing sync. I wish I could deselect both of these preferences, but if I do, the BES server just puts the "undone" item right back on my blackberry thinking the item was mistakenly deleted.