[try it again]
hi Simon,


The problem is that all the old 1.2G and earlier kernels turn off the
write-enable bit for the disklabel area on the `last close' of the
disk partition.  On these kernels, disklabel -W opens the raw
partition, tells the kernel to make the disk writable, and then closes
the raw-disk partition.  If that' the  last close (refcount goes to
zero) the kernel turns the label-write bit off again. So  dd'ing the 
label of the diskimage fails.

Hence my and Michael's suggested workarounds: starting a dd to read
from the raw-disk partition and suspending it (I forget who first
suggested this) or Michael's suggestion of mounting the disk
(works if there's a valid filesystem on the `c' or `a' partitions.)

This probably deserves to go in the FAQ, (hint, hint :-)...