Great plug-in. Does what it needs to. If you ever expand it, please consider adding a similar check for post title and tags. I manage a friend's blog and he routinely forgets categories and tags, but sometimes even forgets to put a title on his blog entry.
Also, I manually edited the pop-up text. An option within the admin control panel would be a touch, too. I re-wrote yours to point out where the category box is.