If you are upgrading OpenSolaris installations prior to build 93 to the latest, remember to follow the instructions carefully:
http://opensolaris.org/os/project/indiana/resources/rn3/image-update/
For snv_86 rc3 upgrades, you need to follow right through steps 6-9. Failure to upgrade the grub information results in an unusable image. Thanks to my colleague Thorsten for the tip!