Mark:

I assume the partition does not appear in /proc/partitions as well?

Regarding "I am reporting from the exact same USB disk under grml 2.6.18" -> so 
does the usb disk work with older grml versions, right?

What do you mean with "Same failures running 'toram' and normally."?

"The disk did show up as ATA/IDE under udev disk-by-id.  Obviously that's 
wrong." - well, disk-by-id is just one of several methods udev provides. So it 
would be interesting whether the disk is present at all.

Which partition schema do you use on the disk?

thx && regards,
-mika-