The ingredients

• One Apple Powerbook G3, Wallstreet edition with 64Mb RAM, 2Gb harddrive, CD ROM drive and ethernet.
• One Debian Sarge PPC 'net-inst' CD
• One network with an AFP server (or other file transfer method)
• One set of apt-sources
• One copy of BootX
• One Mac OS install CD - I used 9.1, but you may wish to substitute another flavour

The procedure

1. Since it's a old wallstreet you'll likely find you can't just boot off the Sarge CD. Or install a proper boot loader (hence BootX). So to begin with, boot off the Mac OS install CD so you can partition ('initialize' in Macspeak) the harddrive.
   Using DriveSetup you select the harddrive, and then pick some partition scheme(s). You can adjust the proportions to suit your taste, but I opted for a 50Mb partition for Mac OS, 256 Mb swap, and the rest for a single root partition for linux. (I ended up with a bunch of tiny partitions that  I assume are for some arcane apple magic - means my root partition ends up as /dev/hda10).
2. Install Mac OS on your hfs partition. (50Mb isn't enough for Mac OS, so my install stopped part way through, but I had enough for it to start :)
3. Boot back into your newly installed MacOS, get BootX, unpack it and install it as suggested by the instructions (Control Panel, Extensions and a folder for the kernels)
4. From the Sarge CD in the folder /install/powerpc/ copy the vmlinux and initrd.gz files into the BootX kernel image directory, run BootX, select vmlinux as the kernel, and initrd.gz as an initial ram disk (in the Options... box) and boot into linux using the Linux button
5. Mix your favourite combination of Debian options in the usual Debian Installer way, but be careful not to scramble the partition table, or you may find Mac OS won't boot anymore.
   Also, if you select ssh as an installer component you can use scp in a later step to copy the kernel and initial ram disk out of your debian install and back to Mac OS for use with BootX.
   I opted for an ext2 main partition, but it's possible that was in a vain attempt to use quik. If/when Debian Installer offers to install a bootloader (like quik) don't let it. The poor old wallstreet G3 can't cope with spicy sauce like quik.
6. Once most of the install is done, drag out the spare terminal (Alt-F2) and use your trusty file transfer utility to transfer the files /boot/vmlinux (which will be a symlink to something like /boot/vmlinux-2.6.8-powerpc) and /boot/initrd.img (which will be similarly linked to /boot/initrd.img-2.6.8-powerpc) somewhere else. They need to end up in the BootX kernels folder to be used for booting.
   I copied the files to another handy debian box using scp (using the installer ssh component), and from there to a Windows 2000 server which had Services for Macintosh installed. Finally, from Mac OS, I could retrieve the files from the Windows server.
   This was the key step I had trouble finding - no combination of kernels and kernel boot parameters did the trick, but using the image and initrd (rather than setting root partitions) has the lil powerbook booting repeatedly (and so far, reliably!)
   There are, of course, other options for getting these files into the MacOS partition. Various pages about Ubuntu (as I recall) talk about mounting the HFS partition directly, and copying the files across. The Sarge install doesn't appear to include HFS support, however.
7. Reboot into Mac OS, set the BootX options to use your newly acquired vmlinux and initrd.img, and you'll be simmering away!
8. Tweak the kernel parameters as desired.
   To get the full screen's worth of video I found adding
   video=atyfb:vmode:14,cmode:8mclk:89
   to the kernel command line quite beneficial.

The proof of the food is in the eating!

At the end of the day, it's what you do with it that counts.

Since my base was a 64Mb Powerbook, after finishing the Debian setup I disabled all the non-essential bits. Turning off exim4 saved quite a bit of used RAM.

The final destination of this wallstreet is hopefully an MP3 player, but that's really another article :)