Ok this is now live and ready for testing on the demo site as long as CKeditor is enabled. You will see a little lock icon next to html content instances. If you click the lock it changes to unlocked and then you can click right in the content and the editor will appear to edit. Note that it respects workflow so if the dropdown in the toolbar is set to "Work In Progress" editing will create or edit a draft whereas if it is set to "Live Content" it will update the live content. When you click outside of the editor the onblur event of the editor fires and if the content has been changed it does an ajax post to a service page that updates the content. So there is no save button when editing inline. Also if editing creates a new draft it will refresh the page so that the workflow icons appear in the title bar of the content instance after the draft has been created. When updating live content or an existing draft it does not reload the page after editing, only when a new draft is created.
Depending on how well this works out and feedback we may look at doing this in other features, but for now it is only implemented in the Html Content feature.