Currently the visual editor does not support the HTML5 Section elements: article, section, aside, nav, hgroup, header & footer. As a matter of fact, it clobbers them when switching from source view to the rich view.
Because of the clobbering, it is impossible to add any HTML5 elements when switching between the two modes. This makes me quite sad as I adding sections to long pages really helps with the layout and organization.
I have also made a ticket for this issue.