Hi Tom,
As far as I know, Debian 6 does not include any EFI bootloader, so it won't be bootable by our EFI implementation. If you want to use Debian 6, you'll need to use BIOS.
Out of curiosity, what is your interest in running Debian 6 on EFI? I'm one of the "virtual EFI" developers, so I am always eager to know more about what people are doing with it. 
Cheers,
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Darius