I believe that you can configure Outlook to use your personal contact list as the first to check for auto-complete.  I don't think it makes sense to sync to the global address book as syncing is intended to be a personal thing.  Not all contacts need be sync'd for all users.

We have a lot on our plate right now, so it will be some time before we can create an offline client.  When we do, we will likely leverage the Microsoft Sync Framework http://msdn2.microsoft.com/en-us/sync/default.aspx.  We would also prefer to create the client using WPF.  Although a Windows Forms implementation would be straight-forward, a WPF implementation would be more forward thinking. 

On the positive side, SplendidCRM is mostly data driven.  With the latest release, even the Save/Cancel buttons are driven from data.