The div you are looking for is a page/html_wrapper block being added via the cms.xml layout file.
Here is the excerpt taken from app/design/frontend/base/default/layout/cms.xml:
First option would be applicable if you use local.xml:
<cms_page>
<reference name="content">
<action method="unsetChild">
<alias>cms.wrapper</alias>
</action>
<block type="cms/page" name="cms_page"/>
</reference>
</cms_page>
<cms_index_index translate="label">
<reference name="content">
<action method="unsetChild">
<alias>cms.wrapper</alias>
</action>
<block type="cms/page" name="cms_page"/>
</reference>
</cms_index_index>
change std
<block type="page/html_wrapper" name="cms.wrapper" translate="label">
<label>CMS Content Wrapper</label>
<action method="setElementClass"><value>main-holder</value></action>
<action method="setAttribute"><param1>html_tag_name</param1><param2>section</param2></action>
<block type="cms/page" name="cms_page"/>
</block>
<cms_page>
<reference name="cms.wrapper">
<action method="setElementClass"><value>content</value></action>
<action method="setAttribute"><param1>html_tag_name</param1><param2>section</param2></action>
<block type="cms/page" name="cms_page"/>
</reference>
</cms_page>
Second option would be applicable if you copy the base layout files over:
Copy cms.xml from app/design/frontend/base/default/layout/cms.xml to app/design/frontend/your_package/your_theme/layout/cms.xml and edit the first layout snippet from above to the following:
<cms_page translate="label">
<label>CMS Pages (All)</label>
<reference name="content">
<block type="core/template" name="page_content_heading" template="cms/content_heading.phtml" />
<block type="cms/page" name="cms_page" />
</reference>
</cms_page>