The below describes how to use GRUB Legacy (GRUB 1). Currently it is recommended to use GRUB 2 instead, via the package grub-pc, as described at IntelMac/BootLoader, but the below is kept for reference.

Configuring GRUB Legacy

The GRUB included with Debian 4.0 Etch (GRUB 0.97-10 or greater) is compatible with Intel Macs if you configure proper MBR/GPT hybrid. Although it was possible to install GRUB during the initial d-i install with many manual fiddling processes of MBR/GPT synching, I recommend you to install LILO initially as above and install GRUB to /dev/sda3 and/or /dev/sda4 later for the ease of process and ease of updating kernel. "rEFIt " on OS X does not like anything other than MSDOS or HFS+ for file system description in GPT table when executing gptsync to create matching MBR record. So I marked Linux partitions as MSDOS before executing gptsync. I used both disked for GPT and fdisk for MBR. Once you marked the MBR with the proper file system type (eg. 83), GRUB is happy to boot Debian by looking into FS and finding files.) Here is an example of /boot/grub/menu.lst for dual booting 486 and amd64 systems:

(Linux kernel is GPT-aware thus the swap partition maybe located in places such as /dev/sda7)

You can use grub to set this up for you. After installing grub (see below), simply run grub-install and update-grub to generate /boot/grub/menu.lst:

grub-install "(hd0)"
update-grub