My existing application is a .NET 3.0 winform app that talks to a WCF service which in turn updates a MS SQL Server database; I can make changes at all spots (Windows client, WCF, server). I've had an inquiry into the possibility of allowing users to work offline and later sync their data back up. I have concerns about stale data mostly as the data is not "owned" by them and another user could have updated the same record while they were offline. I also use NHibernate for my ORM.
What are your thoughts as to the best implementation to add a sync feature?