I don't have access to the machine for a few days to come. But I wanted to thank you all for your input. By now, I think I figured out what went wrong. It actually is a bug in the applet. It offers you to 'Save X Configuration', and I clicked 'Save' to the default value of /etc/X11/xorg.conf. I thought it was some setuid-process, but it isn't. It simply does not save the file at all, and neither does it complain, nor ask for the root password. (I tried everything on another machine, also nv70.)
Workaround: Save to your $HOME and then copy the file to /etc/X11, and you're done. I'll check the Screen Resolution / dtlogin (thanks, rugrat), as well as gconftool (thanks, dkenny) when I have access; for further clarification. This message posted from opensolaris.org
