gptzfsboot doesn't like change (failure after swapping drives)

I've been playing around with building an 8.0BETA2 system with
everything on a single zfs filesystem (I'll get fancier later) on a
zpool that is a 4 disk raidz.  I'm using GPT partitions and glabels so
that I can move the drives around w/out drama.

Things work well, but if I swap a pair of drives then try to boot I
get the message that other folks have reported:

  ZFS: i/o error - all block copies unavailable.
  ZFS: can't read MOS
  ZFS: unexpected object set type lld
  ZFS: unexpected object set type lld

Then a couple of boot: prompts.

If I boot off of the 8.0BETA2 media I can import the pool, even with
the drives in different slots.

When I put the drives back into their original slots (verified by
booting the USB stick and checking with glabel status) I still can't
boot off of them, which surprised me a bit.

Can anyone suggest something that I might be able to do to get a
system in this state to boot?  I've tried importing and exporting and
importing the pool several times.

g.