Great news: After several weeks of investigating, Bart finally figured out what made the driver for the AMD Geode GX/LX chipset fail on recent X.org servers: it turns out that, starting with X server 1.3, changes under the hood took place that affect a number of products booting off a General Software BIOS, making the whole hardware freeze as soon as a DDC probe is attempted. Bart promptly produced two patches against the x86emu component of X server 1.3 that fix the issue. Hurray!
Ubuntu/Gutsy packages of the patched X server 1.3, plus current AMD 220.127.116.11 drivers, are available in my PPA.
For Debian/Lenny, the same packages compile from Ubuntu sources as-is. Meanwhile, those of you running Debian/Etch only need to compile the Ubuntu sources for the above AMD driver against the X server 1.1 available in Etch.
This should finally allow people developing distributions for LTSP or for the OLPC to use standard Debian packages.
The next step is to produce patches against the X server 1.4 in Debian/Sid and Ubuntu/Hardy, and then get the fix merged into the upstream X.org tree before X server 1.5 is released. We're already working on it, but help is always welcome.