I did not do much research on this, so possibly there are even more simple solutions. But if you have OneNote 2010, you probably also have Word 2010 installed, so no need for extra tools.
Once you have configured Word to publish to WordPress it is pretty simple.
From your OneNote topic select File, Send, Send to Blog
Now this item will open in Word. After for example adding a category, select Publish (or Publish as Draft …)