Thanks for the link, that's helpful. That method did work, although there were a couple of things I didn't like about it - the setting appears on the profile preferences rather than display prefs page, and it isn't a radio button. I ended up adding an additional field to the User_Profile table (USER_TOPIC_PREVIEW), and made the appropriate changes on editdisplay.tpl, editdisplay.inc.php, changedisplay.inc.php, and postlist.inc.php
Hopefully I didn't miss anything but it seems to work well.
Andrew