freebsd stable - Nov 2009 - ZFS panic "solaris assert: sm->sm_space" loses pool on RELENG-7

Sometime on sunday our main server paniced with the following error:

panic: solaris assert: sm->sm_space == space (0x5e45000 == 0x5e45600), file:
/usr/src/sys/modules/zfs/../../cddl/contrib/opensolaris/uts/common/fs/zfs/space_map.c,
line: 361

I did some goolging and found a couple of refereces to other people who
have seen this. Both of them, however, could not recover the pool and
needed to restore all the data from backups (which I am in the process of).

Soes anyone know anything more about this ? Specificly if it a known
rpoblem which is fixed in 8.0 ? I couldn't find a PR of any kind, but
the fact that a machine can spontaneously loose all it's data from
a set of filesystems worries me greatly.

cheers,

-pete.