I am running Windows 7 64-bit and Office 2010 64-bit. WMDC syncs with my old Windows Mobile 5.0 device, however, I have all my sync options set to sync directly with Exchange, not with Outlook. (Exchange has to be set up for Activesync)
I still get the pop-up (No default mail client) when I first connect my mobile device, but I click ok and never see it again, until I reconnect. At least this will hold me until MSoft updates WMDC to be a true 64-bit sync program.
Hope this helps....I haven't seen this posted anywhere else... (and I've looked)
By the way, MSoft, please get working on the 64-bit version of WMDC. (And make sure to show WM 5.0 some love when you do.....)