I've just updated my home installation of OpenSolaris 2008.05 with all the snv_90 bits (using my own instructions) and it's gone all quite smoothly. Don't try it if you're on dial-up or have a small bandwidth cap as the update downloaded is just short of 1GB of data.

For those interested, he's the output from the whole process (captured after the event):

First, "zfs list" before we performed the update:

col@opensolaris:~$ zfs list
NAME                                 USED  AVAIL  REFER  MOUNTPOINT
rpool                               2.28G  6.58G    55K  /rpool
rpool@install                         16K      -    55K  -
rpool/ROOT                          2.27G  6.58G    18K  /rpool/ROOT
rpool/ROOT@install                      0      -    18K  -
rpool/ROOT/opensolaris              2.27G  6.58G  2.26G  legacy
rpool/ROOT/opensolaris@install      5.19M      -  2.22G  -
rpool/ROOT/opensolaris/opt          3.60M  6.58G  3.60M  /opt
rpool/ROOT/opensolaris/opt@install      0      -  3.60M  -
rpool/export                        8.73M  6.58G    19K  /export
rpool/export@install                    0      -    19K  -
rpool/export/home                   8.71M  6.58G  8.69M  /export/home
rpool/export/home@install             18K      -    21K  -
col@opensolaris:~$

And now for the action (my root disk is mirrored, hence the two GRUB 'installs')...

col@opensolaris:~$ pfexec pkg image-update
DOWNLOAD                                    PKGS       FILES     XFER (MB)
Completed                                533/533 13882/13882 1172.63/1172.63

PHASE                                        ACTIONS
Removal Phase                              3109/3109
Update Phase                             13792/13792
Install Phase                              5837/5837
A clone of opensolaris exists and has been updated and activated. On next boot the Boot Environment
 opensolaris-1 will be mounted on '/'. Reboot when ready to switch to this updated BE.
col@opensolaris:~$ pfexec mount -F zfs rpool/ROOT/opensolaris-1 /mnt
col@opensolaris:~$ pfexec /mnt/boot/solaris/bin/update_grub -R /mnt
Creating GRUB menu in /mnt
Installing grub on /dev/rdsk/c3d0s0
stage1 written to partition 0 sector 0 (abs 16065)
stage2 written to partition 0, 264 sectors starting at 50 (abs 16115)
Creating GRUB menu in /mnt
Installing grub on /dev/rdsk/c3d1s0
stage1 written to partition 0 sector 0 (abs 2016)
stage2 written to partition 0, 264 sectors starting at 50 (abs 2066)
col@opensolaris:~$ pfexec umount /mnt

And "zfs list" after the update...

col@opensolaris:~$ zfs list
NAME                                                        USED  AVAIL  REFER  MOUNTPOINT
rpool                                                      3.60G  5.25G    60K  /rpool
rpool@install                                              19.5K      -    55K  -
rpool/ROOT                                                 3.59G  5.25G    18K  /rpool/ROOT
rpool/ROOT@install                                             0      -    18K  -
rpool/ROOT/opensolaris                                     5.31M  5.25G  2.26G  legacy
rpool/ROOT/opensolaris-1                                   3.59G  5.25G  2.37G  legacy
rpool/ROOT/opensolaris-1@install                           5.26M      -  2.22G  -
rpool/ROOT/opensolaris-1@static:-:2008-06-11-16:36:44      3.83M      -  2.26G  -
rpool/ROOT/opensolaris-1/opt                               3.60M  5.25G  3.60M  /opt
rpool/ROOT/opensolaris-1/opt@install                           0      -  3.60M  -
rpool/ROOT/opensolaris-1/opt@static:-:2008-06-11-16:36:44      0      -  3.60M  -
rpool/ROOT/opensolaris/opt                                     0  5.25G  3.60M  /opt
rpool/export                                               10.2M  5.25G    19K  /export
rpool/export@install                                           0      -    19K  -
rpool/export/home                                          10.2M  5.25G  10.1M  /export/home
rpool/export/home@install                                    18K      -    21K  -
col@opensolaris:~$