This plugin by Shivendu Madhava is one of my favorites. Small, simple and adds stylish look to your posts/pages. It is so simple that my review consists of examples only!
Delink Pages 1.1 by Jennifer Stuart is now available.
There were some issues with first release of the plugin. Jennifer addressed them all in this new release... and more.
Recently I found Delink Pages 1.0 plugin by Jennifer Stuart. It promises something I was looking for:
This plugin will allow you to specify certain pages to not be linked when wp_list_pages() is used in a theme. You might want to do this if you want a header for a series of subpages, but don't specifically want that header to be a real page itself.