Fellow Ted Pattison Group instructor and SharePoint MVP Scot Hillier just had an article posted on MSDN. It's the first sample I've seen that leverages the CMIS specification announced by Ryan on the SharePoint Team Blog a few months back which essentially is an ECM interop spec so various apps can talk to each other more easily.
His paper shows how to implement a document library in SharePoint but using a external repository that conforms to the CMIS spec. This isn't some thrown together sample... you're looking at a collection of Web Parts, a slick Silverlight UI, custom WCF service, custom protocol handler... you name it. Here's a diagram of the architecture:
Scot has also uploaded the entire project that he discusses in the MSDN article to MSDN Code Gallery for you to download a kick the tires on... awesome resource!
» Scot Hillier: Integrating External Document Repositories with SharePoint Server 2007» MSDN: Integrating External Document Repositories with SharePoint Server 2007» MSDN Code Gallery: Integrating External Content Repository with SharePoint Server 2007