I would like a CMS user to be able to mark one certain page type "featured". I'm assuming in the model you would add a field to the $db array called something like 'IsFeatured' => 'Boolean' and make the default value 'false'. Then you would add a new checkboxfield somewhere within the Root.Content.Main tab. But I only want at most one item in the entire site to be marked as a featured item. So how would I make sure that when I save that any previously marked item is deselected? And I need to differentiate between saving Live and for Draft as well, correct?

You need a custom attribute