I've got this issue too.
The theme is TwentyTen with sidebars and widgets.The version of WP is 3.4.
It works only for posts with tags, but not posts and pages with custom field.
Please check if the following settings are correct:
in the widget, the following options are check:
>Title and Tags form left empty
>Get tags and title from custom fields. You need to set the custom field for each post/page.
while i edit a page, i put some text in the Widget Title and Widget Tag box , this is the Custom Fields, right? But when i show that page, nothing appears in the sidebar.
Thanks for your help and great works!