Ah, the price we pay for being on the bleeding edge. Windows Mobile Device Center is how Windows Vista wants to sync with your Windows Mobile device - it's a leg up on the old ActiveSync program in pretty much every respect but one - file syncing doesn't seem to work for you WM6 Standard Edition folks. Never fear, though, Brighthand is reporting that Microsoft feels your pain and will get WMDC fixed up right-as-rain in a little less than a month.
Currently, Windows Mobile 6 users can synchronize files between PCs and smartphones with Active Sync 4.5. Microsoft will update Windows Mobile Device Center (WMDC) on Windows Vista PCs to support file synchronization with Windows Mobile 6 by mid-June