Are you sure User1 really has become the owner of the post? I have problems with a new install of WP 1.5 where all user posts (except for admin posts) become owned by the user with the lowest user id number.
When a user tries to edit comments on someone else’s post, I think the error message is as you described. When a user tries to edit someone else’s post, the error message I get is “You are not allowed to edit this post”.