According to Microsoft, recent Vista builds (RC1, RC2) support booting
on a x64 machine with EFI firmware, such as a MacPro. Looking inside
the ISO images of Vista x64 RC1 provides evidence for this (presence of
an EFI folder, of a bootmgr.efi file at the root level, etc.).
According to http://www.microsoft.com/whdc/system/platform/
firmware/efibrief.mspx, the Vista ISO uses a second El Torito boot
catalog entry to point to a FAT32 filesystem with the Vista bootloader
on it, which then boots the Vista installer (itself on an UDF filesystem).
However, it seems that Apple's firmware (and rEFIt) does not see this
entry and boots using the first boot entry (which is the legacy one).
Assuming it is operating under a BIOS, the DVD installs the legacy, non
EFI-supporting version of Vista.
Would it be possible for rEFIt to support the EFI boot entry on the Vista
DVD, making a native Vista installation possible ?