Update - Check out the all new Custom Sidebar Pro for all your widget, sidebar and footer needs.
Since yesterday we have a new stable version of our beloved WordPress, version 3.2 nicknamed Gershwin after a great jazz musician. It comes with a lot of nice features, a new brand administration area, huge performance improvements, security patches and bugs solved.
I've been trying the betas and RCs to be sure that Custom Sidebars is compatible but unfortunatelly the official version broke the sidebars selection on pages, because it fixed a misuse of the property "publicly_queriable" on custom post types that makes the pages be not "publicly_queriable" anymore. Since Custom Sidebars checked if the post type was publicly queriable to add the selection box to the edit form, it was completely dissapeared for much of you.
Anyway, everything can be fixed but death, so here you have a new version of the plugin that behaves as expected. Enjoy it.