In SharePoint 2010, if you add a new SPNavigationNode as a child to a Heading link, then the type of the new link will automatically be Link:

If you however add it as a root level node, then the type will be Heading:

Use the following code to add a root level node as type Link:

using (var site = new SPSite("http://myServer"))
{
	using (var web = site.OpenWeb())
	{
		var newNode = new SPNavigationNode("Root Level Link", "http://www.ms.com", true);
		newNode = web.Navigation.QuickLaunch.AddAsFirst(newNode);
		newNode.Properties.Add("NodeType", "AuthoredLinkPlain");
		newNode.Update();
	}
}

newNode.Properties.Add(“NodeType”, “AuthoredLinkPlain”) is the magic line that makes it works.