zfs discuss - May 2010 - zfs destroy -f and dataset is busy?

We have a pair of opensolaris systems running snv_124.   Our main zpool 
''z'' is running ZFS pool version 18.  

Problem:
#zfs destroy -f z/Users/harrison at zfs-auto-snap:daily-2010-04-09-00:00
cannot destroy ''z/Users/harrison at
zfs-auto-snap:daily-2010-04-09-00:00'':
dataset is busy

I have tried:
Unable to destroy numerous datasets even with a -f option.
unmounted the filesystem and remounted it same problem
exporting and importing the zpool
The zpool scrub completes without errors:
12:51pm taurus/harrison [~] 182#zpool status z
  pool: z
 state: ONLINE
 scrub: scrub completed after 17h32m with 0 errors on Sun May  2 
18:47:38 2010
config:

        NAME         STATE     READ WRITE CKSUM
        z            ONLINE       0     0     0
          raidz2     ONLINE       0     0     0


Any suggestions would be greatly appreciated.

Thanks in advance,
    -C