Custom post types columns strange issue

Worked it out! Since WP version 3.1 the hook for adding columns to custom post types has changed.

Before 3.1 it was:

add_action("manage_posts_custom_column",  "testimonials_custom_columns");

After 3.1 you have to use this:

add_action("manage_testimonials_posts_custom_column",  "testimonials_custom_columns");

So you have to specify the custom post type name in the manage_post_custom_column hook.

Hopefully this answer will save someone some time.