Thank you for suggesting this feature. I have logged it in our feature request system and updated your Telerik points. For this feature we will need to wait at least until Sitefinity 4.0 because current data access layer (which we will change) would cause dramatic performance issues when it would query to find items in use. With the new DAL I am rather confident we will be able to offer much more extensive reporting features, the one you've suggested, being one of them.
I hope you'll find this information helpful. Let us know if there is anything else we can do for you.
the Telerik team
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