Lead Developer
Interesting – by default, WordPress requires you to regenerate the thumbnails when new sizes are added to the_post_thumbnail() function.
However, in the latest version of Page Header/Blog add-ons, I’ve included a script which detects if these sizes change, and regenerate the images automatically.
If you’re not using the add-ons above, you’ll have to manually regenerate using one of the plugins available (Force Regenerate Thumbnails for example).