I believe I've figured out what the problem was, at least with our configuration. We have both Solaris 10 update 4 and update 5 on the jumpstart server, and the image being served over tftp was from u4 while the stuff served over NFS was from u5. Deleting the content on the jumpstart helper server and re-running setup_install_server appears to have fixed things.
Thanks for the suggestions. Will
