The currently recommended image for Raspberry Pi is the Debian based Raspbian distro.
Playing in this sandbox is a new experience for me. It feels like the Linux I'm used to, but there are some subtle differences. I'd thought I might share my thoughts and experiences.
I'm used to playing with Gentoo/Linux on x86-64 hardware. My desktop, laptops and home servers all use the amd64 instruction set. Even my tablet can understand amd64. This makes it really easy to create Gentoo binpkgs, and share /usr/portage over nfs.
With a raspberry pi, the first thing I did was to do a stage3 install of Gentoo for the armv6-hardfp architecture. It took me a little less than a week to get sound, X and a few useful programs such as fluxbox, mplayer and synergy (I don't have a spare keyboard/mouse to use, don't need it anyway). Of course, mplayer could just about play audio files, and video was entirely unacceptable. Raw Gentoo is not optimized for the Pi, especially since nothing is hooked into the GPU.
I did learn some lessons. How to setup cross-compilers (crossdev -S). How to boot a pi (especially interesting since the Pi has no BIOS, or CMOS). Dabbling with Gentoo first was time well spent.
The Raspberry Pi has no onboard firmware, or anything to store state at all. The bootloader has to be provided on the SD card.
From what I can tell, bootcode.bin is a binary blob that tells the GPU what to do. start.elf comes in many flavours, usually to tell the RAM split between GPU and ARM host processor. Finally, kernel.img can be cross-compiled from vanilla/gentoo sources, or use the raspberry pi patches for hardware compatibility.
The final kernel.img is created by another tool called mkimage, which tacks on an extra 32k of magic to the compiled kernel image.
The lazy way is to create the fat filesystem, mark it bootable then copy the
contents of https://github.com/raspberrypi/firmware/tree/master/boot into it.
Of course, don't forget to also place the kernel modules into
uname -a on whatever root filesystem is used.
There is no faffing with grub, and editing the boot process can be done from windows.
It would also seem that the only way to boot a Hard Drive would be to boot from the SD card, then really boot from a usb hdd. This isn't that hard, since it should be possible to replace kernel.img with a chosen bootloader, instead of a kernel.
I'm now running the raspbian image. I have ssh starting on boot, so I can login and fiddle without a Keyboard/Mouse (or HDMI Monitor which I don't have at home). I apt-get install'd synergy and screen so that I can use a spare monitor at work. (The raspberry pi serves as a great nagios monitoring display).
Hexxeh has put together a Chromium build, which I find uses less of the (precious) CPU cycles than Midori. So that has become by full-fat browser.
It looks like I will be staying with Debian for a little while. Fedora (from the QtonPi project) seemed unfinished, they're moving to OpenSUSE anyway. Ubuntu won't support the Pi because ARMv6 is too ancient for Canonical. Gentoo worked well, but I probably won't try that again until I can get GPU drivers/libraries in ebuild form. Of course, if the Chrome OS (which is essentially Gentoo) port is finished, then I'd gladly try that too.
One last thing that I've been attempting is to read the Debian Reference and Debian Handbook. Once I figure out how to do common tasks (def: tasks that I find common) such as kernel recompile/reconfigure, tarball/git repository source code builds then I will be much happier.
From 5 minutes of googling, any tarball or repo with a debian/rules file (pretty much everything in the floss world) can be dpkg-buildpackage -us -uc'd into a_ ../$package-$version.deb_. Then dpkg -i it into the live system.
Oh, how I miss epatch_user.