This is probably really stupid, but it took me about 15 minutes of prodding and googling to figure out, so here it is. I have a Blade 1000 upon which I've installed an older release of Solaris 10, with the intention of testing a flash upgrade before I try it "for reals". Problem: it wouldn't automount my August 2007 release of Solaris 10 on DVD.
Solution:
svcadm enable smserver
was the final step, although
svcadm enable volfs
may be required as well. After that, it will mount at /cdrom.