freebsd stable - Jun 2009 - ZFS jumps from slice to partition on upgrade (???)

I just bumped from:
  FreeBSD 7.2-PRERELEASE #1: Wed Mar 18 00:55:27 PDT 2009
to:
  FreeBSD 7.2-STABLE #2: Wed Jun 10 18:27:30 PDT 2009

I rebooted and my zpool was WHACK'd.  I was getting these errors in 
/var/log/messages:

Jun 11 16:54:55 crepe3 root: ZFS: vdev I/O failure, zpool=tank 
path=/dev/ad6s2c offset=481721292800 size=1024 error=5
Jun 11 16:54:55 crepe3 root: ZFS: vdev I/O failure, zpool=tank 
path=/dev/ad6s2c offset=481721293824 size=1024 error=5

I had ad0s2 and ad6s2 in my zpool mirror, but after the upgrade zfs read 
the devices as:
  ad0s2 and ad6s2c.

So I dropped the 'c' parition and reattached the slice:
  # zpool detach tank ad6s2c
  # zpool attach tank ad0s2 ad6s2

and now my 'zpool status' shows it resilvering...

   pool: tank
  state: ONLINE
status: One or more devices is currently being resilvered.  The pool will
	continue to function, possibly in a degraded state.
action: Wait for the resilver to complete.
  scrub: resilver in progress for 0h22m, 21.91% done, 1h21m to go
config:

	NAME        STATE     READ WRITE CKSUM
	tank        ONLINE       0     0     0
	  mirror    ONLINE       0     0     0
	    ad0s2   ONLINE       0     0     0  82.2M resilvered
	    ad6s2   ONLINE       0     0     0  19.0G resilvered


QUESTION: Why the heck would ZFS latch onto ad6s2c partition instead of 
the ad6s2 slice?  I've rebooted many times before without issue... it 
seems the upgrade bonk'd things.

Hopefully when this is all done, I can run zfs upgrade...  anyone have 
trouble with "zfs upgrade"?

Rudy

History for 'tank':
2008-01-17.23:07:08 zpool create tank mirror ad0s2 ad8s2
2008-01-17.23:40:35 zfs create -o quota=100g tank/test.monkeybrains.net
... many more creates/deletes ... then the zfs freakout ...
2009-06-11.16:43:33 zpool clear tank
2009-06-11.16:43:59 zpool scrub tank
2009-06-11.16:48:58 zpool scrub -s tank
2009-06-11.16:54:56 zpool detach tank ad6s2c
2009-06-11.16:55:57 zpool attach tank ad0s2 ad6s2


Maybe having the device jump from ad8s2 to ad6s2 freaked out zfs?

Any thoughts, or will the Internet tube swallow this post silently?

Rudy