Turns out the one thing I forgot was to change the OS Version on the VirtualBox settings for the VM from Opensolaris to OpenSolaris 64-bit. After that, it booted 64-bit by default as expected.
I also added the full screen resolution (1920x1080 for me) to the xorg.conf, and that's working nicely too...using it right now. It's fast enough to be usable for relatively undemanding purposes, which is to say pretty darn good for a virtualized environment. Once OSS is installed, audio works, and for example, a Flash* video (small, or up to about 1/4 of screen) plays well enough, but forget about trying to play at full 1080p; that won't play fast enough, not that I really expected it to. *had to install that too, of course -- This message posted from opensolaris.org
