I had been trying to install Linux on a 1GB USB flash drive connected to my MacBook Pro (MBP) since the past few months. Although there are several places describing how to do it using either Boot Camp or by resizing the hard disk partitions, my sole requirement was no changes to the Mac OS X installed on the hard disk.
Boot Camp was out of question as its main aim is to help install Windows by providing drivers for the Mac hardware. The other option was using rEFIt and resizing the hard disk. I had tried this earlier with mactel-linux patches for the kernel and was able to get a login prompt but no video. That process involved several steps using debootstrap and compiling the kernel with the patches.
However, with the release of Debian 4.0 (Etch), the debian-installer also got a lot of attention and installs a relatively new 2.6.18 kernel. So armed with a netinstall d-i CD (23 April build, 26th doesn't recognize network card) I was able to install Debian Testing (Lenny) on the USB disk with XFCE4 as the desktop environment with 130MB of swap space partition and 157MB to spare after installing dillo web browser.
The installation of Lenny on the USB flash disk has not required any change on the internal MBP hard disk and the Mac OS X installation is completely oblivious of the Debian install. If I don't press the "Option" button at boot time, Mac OS X boots up, otherwise with the USB connected it asks which OS to boot.
Next post gives the link for installation steps. The process was based on information gathered mainly from the Debian wiki's Macbook and Macbook Pro pages, and also here.