X64 is simply more memory-hungry than X86. I don't think it makes sense on a portable device.

Office 2010 comes in a 64-bit version largely for marketing reasons. An exception may be Excel 2010 for people who use Excel for insane things and need more rows.
Note:

In a default installation of Office 2010, the 32-bit version is installed, even on 64-bit systems. You must explicitly select the Office 2010 64-bit version installation option.
http://msdn.microsoft.com/en-us/libr...ffice.14).aspx

The same article goes on to describe numerous headaches when using VBA7 in 64-bit Office 2010, such as the controls you lose.