I am experiencing the same problem, but NOT using Twitter Tools and the problem also persists even when I deactivate all of my plugins at once.
I have just noticed this problem since upgrading to 2.9. My database has always been MySQL 5 so MySQL 4 isn’t the problem.
I don’t get the error message if I save a draft with no title. Then I can add the title and save as draft and it works. Also, when I do get the error message it still saves the post and I can see the new post show up when I go to the ‘edit posts’ menu.
Is anyone else having a similar problem unrelated to Twitter plugin? Any ideas?