That should be ok.
You should rewite the uImage and the file_system.jff2 make sure you use
flash_eraseall -j /dev/mtd1
flash_eraseall -j /dev/mtd2
nandwrite -pm /dev/mtd1 uImage.sheeva.20090319
nandwrite -pm /dev/mtd2 ubuntu-9.0.5.Release.jffs2
and make sure You use the same mtparts definition in all you different bootargs
Indeed, I did flash_erase and flash_eraseall before doing nandwrite and I made sure that mtd partition definitions are the same.
Now I am 99% sure that the ubuntu-9.0.5.Release.jffs2 image, taken from the "Download" section of the WEB site, is broken.
Today I am going to check the image from the companion CD (I should have tried that before! But the rule "Online image is always more up-to-date than on a supplied CD" played a bad joke on me, I guess).
On a side note: I tried to mount the older, 9.0.4 jffs image (using mtd emulation on a block device), taken from the site - and it mounted allright and contained all the necessary stuff inside. So the next experiment will be to flash that one and to see if it works.
Will keep you posted on the outcome of these experiments.