If you're not already aware, the R-Pi boots directly from the SD card via the GPU ROM, which loads and executes a bootloader that then brings up the ARM CPU in what eventually becomes a more-or-less standard ARM startup (think of the BCM2835 as a screaming 26 GFLOPS floating-point/matrix-multiplying microsupercomputer that just happens to have a 700 MHz ARM CPU tacked on as a traffic cop to route non-video/audio I/O (USB/Ethernet/SD/GPIO/etc.) between the ports and the GPU, 256 MB POP RAM, etc.
I don't know whether the process is documented yet, beyond whatever may be included as readmes/comments in the existing ports of Fedora, Debian, ArchLinux and now, OpenELEC/XBMC, with Puppy Linux and others reportedly coming, sooner or later. The GPU ROM, its internal hardware details, and assembly source are not going to be released, so, you will need to use the binary blob the GPU uses to boot, and whatever other helper files are needed to make the bootloader happy to kickstart the CPU.
We can hope that, if it's not already clearer than mud from existing documentation, the Foundation or one of the OS partners will be providing the info needed to develop our own OS flavors. However, I suspect this is not a priority, as they're just now getting the very first versions of the Linux ports out into the wild, and there still isn't any hardware to run any of them, and possibly weeks before even the (Randomly) Chosen 10,000 can try anything out on an actual board.
Since secondary (and, eventually, primary) education is the current initial intended audience for the R-Pi, I wouldn't hold my breath for detailed assistance in bringing up a DIY OS, so, unless you can either figure it out from the initial Linux ports, or some of us other end-user unofficial developers are able to do so, it's going to be a challenge-and-a-half. Personally, I would be interested in porting/implementing some experimental OSes that minimize the cruft that inevitably winds up in kitchen-sink general-purpose OSes.
The best things in life aren't things ... but, a Pi comes pretty darned close!
"Education is not the filling of a pail, but the lighting of a fire." -- W.B. Yeats
In theory, theory & practice are the same - in practice, they aren't!!!