A little something I found:
"
For one a limitation set by Apple is that Snow Leopards 64-bit kernel works only on Macs with 64-bit EFI. Technically a 64-bit kernel can be launched by a 32-bit EFI just fine."
Of course I am no longer on Snow Leopard, I am on Lion.
"
Rule
A 32-bit processor can only run 32-bit stuff.
A 64-bit processor can run 32-bit and 64-bit stuff.
(stuff can be system, kernel, applications, drivers, etc)
So you need a 64-bit processor to run 64-bit anything
On a 64-bit processor:
A 32-bit Kernel can run both 32 and 64 bit applications.
A 64-bit Kernel can run both 32 and 64 bit applications.
A 32-bit Kernel can load only 32-bit kexts (kernel extensions).
A 64-bit Kernel can load only 64-bit kexts (kernel extensions)."
Point is
some drivers can be 64 bit and still run on Mac Pro 2,1's, and the kernel could be 64 bit as well, but Apple made the call to prevent it in some models, with both 32 bit EFI's and 64 bit EFI's.
Link