Tweaking WordPress With Greasemonkey
Update: Here are a few more WordPress GreaseMonkey scripts that the author is offering.
wordpressuncheck.user.js is a GreaseMonkey user script that waits until the admin “Write Post” page loads, then goes through and unchecks whatever WordPress chose as the default category. It only deselects when a new post is created, otherwise it would overwrite the categories you already selected for drafts. Its only requirement is that you keep the default “wp-admin” folder.