ZFS "Dataset is busy" error

[Richard Mahlerwein]

I am attempting to test the interation of ZFS snapshots with the Previous Versions capability of Windows.  I installed FreeBSD 8 into VMware workstation.

curie# uname -a
FreeBSD curie.localdomain 8.0-RELEASE FreeBSD 8.0-RELEASE #0: Sat Nov 21 15:48:17 UTC 2009     root at almeida.cse.buffalo.edu:/usr/obj/usr/src/sys/GENERIC  i386

I added a second drive (da1) and created a test pool on it.

curie# zpool history
History for 'tank':
2009-12-30.14:23:08 zpool create tank da1

I installed Samba 3.3 from ports...
curie# smbd --version
Version 3.3.9

Added a super simple entry to my smb.conf to allow user test to get to the shared /tank (which works fine, AFAICT), restarted all services and browsed the share from my windows host.  I dropped in half a dozen text files, then tried...

curie# zfs list
NAME   USED  AVAIL  REFER  MOUNTPOINT
tank  70.5K  2.94G    18K  /tank

curie# zfs snapshot tank at snap1
cannot create snapshot 'tank at snap1': dataset is busy

I've tried google, but it's of not much help given the above strings to search on.
(I also tried the freebsd-fs archives, but I just get this... Note:The archive search index was last rebuilt at  Wednesday, 07 Feb 2007 20:23:54 UTC. Any postings after that will not be found by  a search. Index rebuild is usally done once every 24 hours for  this list. You can use a "View by date" link below to access  more recent postings. )

This is nearly the same setup as I had 3 or 4 weeks ago on a VM while I was testing ZFS itself, and that worked flawlessly. I'm not sure what's different or changed.  I shouldn't have deleted that VM.  :)

Can anyone help point me to where to look for what's going wrong, or for what other information you may need?