fdisk incompatibility between major versions

[Robert Huff]

    Due to a combination of my spectacular stupidity and external 
events, a machine running -Current from late April will no longer boot. 
  Specifically, instead of the loader menu screen I get "Invalid partition".
    Crud.
    Broke out the most recent fixit disk, which is from 7.1 (Jan 2009).
    Boots fine; fdisk finds the correct slice information; bsdlabel 
finds the right partition data; mount / and /usr under LiveFS
    Whew!!!!
    So ... I'm guessing the problem in the boot code is corrupted, and 
if I do

     fdisk -B -b <path to -CURRENT boot blocks> da0

     it should put things back to normal.
     However: when I do the second commit I get

     fdisk: Geom not found "da0"
     fdsk: failed to write sector 0

    This leads to the assumption there is a fundamental discrepancy 
between the two versions.  Not unreasonable, as I know there have been 
changes in the filesystem code since 7.1.
    0) is the diagnosis correct, and will my proposed solution work
    1) if so, is there a way to convince the 7.1 fdisk to Do The Right 
Thing?
    2) if not, what do I do next?

    Respectfully,


                               Robert Huff