Thanks to all of those who helped sort this out. I checked the bug report log and found that a bug had been filed (by Joey Hess) against the installer package for realplayer 8 (this was the debconf problem).
There was also a reply from the maintainer saying that he planned to put a fixed version into proposed-updates. That doesn't seem to have happened yet. So at present there is no installer package for either rp7 or rp8 in potato (despite the fact that the packages database at debian.org says that rp7 is available). I downloaded the RPM version of rp8, used alien to convert it, and installed it with dpkg. All seems fine. My thanks to all who helped, Jim
