The first editions of the Mac Pro (1,1 thru 2,1) were born in 2005 with existsing 32-bit firmware and ML is becoming 64-bit only.
Not that Vista and PCs don't use 32-bit firmware and still have 64-bit kernel. If you ever tried Windows 64-bit you know the trouble we have with that on these models as well.
However, even Windows when it sees EFI and 64-bit OS expects to see Unified EFI aka EFI64 / UEFI 2.x rather than the older EFI 1.x that we have.
You can order Lion 10.7 and use that. Or you can go into the hackintosh route and treat the Mac as a PC almost and dual-boot loader to get there. But you would have to look elsewhere for support and help on other forums.