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?

Still trying to figure this out, anyone have any ideas? Is there some onSave method where you can add custom functionality to the default CMS behavior?

You need a custom attribute

onAfterWrite did the trick. http://www.silverstripe.org/general-questions/show/273211