In the Solution there is a Site Collection Feature which registers two Custom Actions.
Navigate to a ContentType:

The "View usage" CustomAction should be visible:

The urls will be displayed of the Lists where the ContentType is in use:

This solution is not extensivly tested yet.
https://MRCL.svn.codeplex.com/svn/Wss3/ContentTypeUsageSolution/ContentTypeUsageSolution.wsp
WSPBuilder + Visual Studio 2008