just some suggested features, based on my work w/ the WPAlchemy metabox class http://farinspace.com/wpalchemy-metabox/ which I use on almost every project. It has 2 really cool features that I think would be great blended into this plugin.
1. the ability to limit where the additional thumbnails go.... at least by post type... WPAlchemy also can restrict by template, or category, etc. Regular posts might only need 1 thumbnail, but you might need 4 for a portfolio piece.
2. the ability to let the user define how many custom thumbnails need to be added per post. WPAlchemy has a loop called "have_fields_and_multi" then lets the user click a button to add another instance of a meta field. This way a user might add 2 thumbnails for one portfolio piece and 4 thumbnails for another.
cheers! and thanks for the great plugin.