A new version now squashes the following bugs:

• Unable to activate the plugin on web servers running PHP4

• The Post Reader / Contributor / Editor interface was not displaying in the “Write Post” / “Edit Post” form.

Thanks to Julien on the WordPress support forum for reporting the PHP4 issue; my apologies to anyone else discouraged by my failure to run a PHP4 sanity test prior to release.  PHP5 users interested in assigning post-specific roles should also upgrade.