I've been having some problems with the OpenIndiana oi_148 to oi_151a update, but only on one virtual machine in particular. Until today i've been stuck, until I thought to:
pkg set-publisher --non-sticky opensolaris.org
After that, the image-update went fine. I need to learn more of the options for pkg, apparently. I had skipped that before because my other OI machines did not have that non-sticky option set, and the upgrade worked there.