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.
svcadm enable smserver
was the final step, although
svcadm enable volfs
may be required as well. After that, it will mount at /cdrom.