Hi,
I've been digging into the "unknown filesystem type: jffs2" problem that makes boot fail most of the time on XO-1.
First observation, this isn't the message that appears furthest down on the screen, it's about halfway down. The one at the bottom of the screen says "creating devices", making it appear like the hang has happened there, but actually the screen is screwed up at this point and lines being printed are appearing in something like this order:
5 6 7 8 unknown filesystem type jffs2 1 2 3 4 creating devices
I added some initrd debug messages and confirmed that jffs2 is not loaded before the "mount" call is made. The mount procedure (on the kernel side) is therefore responsible for loading the module.
Turning plymouth off (by commenting out the appropriate lines in the initrd scripts) causes messages to appear on-screen in the right order *and* seems to reliably work around the race bug.
At this point I'm having severe difficulty to boot this kernel to add more diagnostics, even after a fresh reflash, even on a 2nd XO, due to an even earlier hang problem (see my "another boot hang" thread).
Daniel