@misiak and @hurrian,

Yup, for some reason I forgot to type "rootfstype=ubifs" as part of the command line. That might explain the different messages from the kernel.

Oddly enough, I have a copy of the roofs on the eMMC (for backup purposes). If I only had a kernel with built-in MMC/EXT3 modules..

Once I get this whole mess cleared I will prepare a combination of U-boot environment, Kernel(s) and backup-rootfs (Maemo, Rescue, Nemo, whatever, the more the merrier (e.g. on eMMC *and* also under MyDocs as a fs image, whatever) so that the N900 will be completely unbrickable even if USB *and* SD card are not functioning.