WP 3.1 has some nice tools for putting thumbnail images on post excerpts now. One trouble I'm having is that images don't always auto crop or scale correctly to make well composed thumbnails.
WP has the ability to edit an image, and crop it proportionally, so this would seem to be a nice solution. But I find that when I crop, or resize an image in the editor, I can't set it as the designated thumbnail. I can put it in my post, but I can't make it the thumbnail.