The Office 2010 team has posted comprehensive information regarding the 64-Bit release of Office, including details on who should install Office 2010 x64:
“If you’re trying to decide between 32-bit and 64-bit Office, you should ask yourself what your needs are. Are you an Excel power user working with huge amounts of data? Do you need to work with file sizes greater than 2 GB? If so, then you would benefit from 64-bit Office being able to utilize more memory. If not, we’re recommending 32-bit Office 2010 as the default installation on both 32-bit and 64-bit Windows mainly due to compatibility with existing 32-bit controls, add-ins, and VBA.”
“Both 32-bit and 64-bit versions of Office are largely indistinguishable, except that 64-bit Office has a much higher memory capacity. As mentioned above, the extra memory capacity comes at the cost of some compatibility with existing extensions to Office, such as 32-bit versions of ActiveX Controls and some 3rd party add-ins, in addition to 32-bit versions of programs that interface directly with Office. New versions of these extensions will need to be obtained, and it will take some time for 64-bit compatible extensions to be made available.
For these reasons, we recommend running 32-bit Office 2010 even on 64-bit Windows operating systems for better compatibility. On 64-bit Windows, more applications and documents may be opened at once, and switching among them will be faster because the machine can have more physical memory for the processes to share. When the 64-bit ecosystem for Office is more mature, you’ll be able to easily migrate to 64-bit Office!”
In the near term, if you want to run ClearContext in Outlook 2010, you should install the 32-bit version of Office 2010. For information on future ClearContext support for 2010 x64, see this forum post.