I know that a decision was made to change the way default post categories are handled in WP 2.1, however I would like to request this issue be revisited.
I would propose adding an additional to the Default post category dropdown in Writing Options titled "[ None ]" or something to that effect. When this option is selected, by default all new posts would belong to no categories and all boxes would be unchecked in the Write Post admin page. However, if a specific post category is selected from the dropdown, it would appear checked in the Write Post admin page.
My main issue is that it's non-intuitive to select a specific "Default post category", yet have new posts appear to be completely uncategorized when composing them. The issue is exacerbated if you select a non-default category in the Write Post page thinking that the default might also apply, only to find that the Default post category is not selected in that scenario after saving or publishing the post.
I can appreciate the desire to not have categories selected by default, but I would strongly argue that the way it's currently implemented is not at all intuitive for new WordPress users.