From Office 2010, native x64 version is included. The funny thing is, if you install an x64 version of Outlook into your windows, no matter windows xp (x64), vista (x64) or win7(x64), you just cannot synchronize the data between Outlook and your windows phones.
It seems that ActiveSync(xp) and Windows Mobile Device Center(vista and win7) cannot recognize the x64 version of Outlook. And, the x64 version Outlook cannot be installed on the operation system which contains any x86 office components.
If you need to sync your windows phones with outlook, you have to install the 32bit office.