Could future versions of WP please allow thumbnails uploaded to the gallery to be globally re-sized? Themes should have the ability to stipulate a thumbnail size, and have WP honor it. This way one theme could call for 32px thumbs, another 128px thumbs, and the correct sizes would be generated on the fly.
Ideally this could be done on an as-needed basis. What I mean by that is that the processing required to regenerate thumbs would happen the first time a user requests said thumb via a theme, and would be stored so that subsequent site hits would have the newly-resized thumbs ready to be served.