We don't even have to use RSD or .sbf files anymore. Why? Because our friends at Motorola also made enhancements to fastboot which allow you to flash >256mb partitions. This means we can overwrite large partitions, such as "system" (/system) and "webtop" (/osh), in addition to the blessed "boot" (kernel+ramdisk).
I've compiled it from the opensource.motorola.com (thanks Motorola!) for linux, it is attached to this post.
UPDATE: Looks like romracer came through for all you people who still use windows, the MinGW version is attached. Thanks romracer!
(After you thank romracer's post you can see this guide)
If the files below (for your 32 or 64 [default] bit system) don't work, you can compile from fastbootNfriends yourself with a string like:
gcc -o fastboot usb_linux.c util_linux.c fastboot.c protocol.c engine.c bootimg.c -lz ../libzipfile/zipfile.c ../libzipfile/centraldir.c -I../include -I../libzipfile -I../mkbootimg
Next post will tell you exactly how to use it to test out the HKTW 2.3.4 that was leaked a week ago which gave us all these great clues!
For me, when Atrix is connected ("lsusb | grep PCS" on Linux) it shows up as ID 22b8:708c if its in Fastboot mode.
The leaked build from Hong Kong / Taiwan was probably not built for a device that is exactly like ours. Test phones can have different hardware. Don't expect everything to work, it's super-duper beta and since we've all only been unlocked 24hrs no one really knows how usable it is! Flash at your own risk... (PS you can 'Skip' Motoblur just by hitting 'Menu' during setup. It won't connect anyway...)
PPS. You can still use "dd" or something dangerous to overwrite your mmcblk0p11 and mmcblk0p12 with new builds, if you are unlocked, but at your own risk.