I've looked at a few of them...they're all happening right away for me. Someone will unsubscribe, and within 10-15 minutes, that info is synced over to SFDC (odd, b/c I thought the sync was usually every 5 minutes). Then, Marketo will sync with SFDC, and the very next line will show (at the same time) a Change Data Value for the unsubscribe field - from True to False. When I called Marketo Support, they said the change was coming over from SalesForce when Marketo was trying to update the SalesForce data.
So it doesn't seem to me that a workflow could be triggering it, but I've asked my SFDC admin to look at that again. A workflow wouldn't work that quickly, right?