Recently, after upgrading my Office to Outlook 2010 64-bit, I plugged my phone in to my standard weekly sync with my Outlook, and up popped a message along the lines of "wrong outlook client". At the same time the sync (or lack thereof) process deleted all my contacts from my phone. After a very quick search I soon dscovered that this is a known problem between WMDC and Outlook 2010 64-bit. The official statement from Microsoft is this:

Question/Issue
"Windows Mobile Device Center (WMDC) no longer syncs with Outlook when 64-bit Outlook 2010 is installed on 64-bit Windows.WMDC also no longer syncs with Outlook if you are using a Click-to-Run installation of Outlook.Upgrading 32-bit Outlook 2007 to 64-bit Outlook 2010 could also result in data loss during first sync with WMDC.

The WMDC will not be upgraded to work with 64-bit Outlook 2010 or Outlook Click-to-Run.

Workaround
"If you need the functionality of WMDC and you have already installed 64-bit Outlook 2010 or Click-to-Run, uninstall Office 2010 and install the standard installation of 32-bit Office 2010.

Otherwise, install the standard installation of 32-bit Office 2010 if you want to continue to use WMDC."

Basically, they released a 64-bit version of outlook and then tell everyone not to install it! and the answer is..wait for Windows Phone 7 that will sync fine. So what do we do in the interim?? I have been forced to now use a 3rd party outlook to Gmail sync, and then sync my phone with gmail, but that is not really a solution.

While they say they are not bringing out a solution, I certainly hope they do, as there are a lot of angry users on the forums currently discussing this very issue!