zfs refuses to load pool on boot, zpool denies import

I'm constantly getting this error on bootup:

ZFS: WARNING: pool 'mypool' could not be loaded as it was last
accessed by another system (host:  hostid: 0x0).  See:
http://www.sun.com/msg/ZFS-8000-EY

As I've put /usr into zfs, I need to reboot into single user and issue
a `zpool import -f mypool' (w/o -forcing, zpool denies to import the
pool). It's strange as this pool has been created on that machine
(probably while having booted from a different disk - I don't actually
remember).

I already imported, exported and re-imported but it didn't help.

Any ideas?

%kenv smbios.system.uuid
ccf8597d-74fe-d511-a413-782ede476378

FreeBSD 7.0-CURRENT #2: Fri Jul 13 19:29:31 CEST 2007 (i386)
csup'd & rebuild today, kernel mostly GENERIC (+ALTQ added)

Thx

Volker