Ironic, the day after I publish the link to the Documentum vs. SharePoint article, Microsoft releases an MSN article on integrating the two :). So, for more information on how to integrate SharePoint 2007 with external document repository systems, such as Documentum, see the links below:
Integrating External Document Repositories with SharePoint Server 2007
Code for Integrating External Content Repository with SharePoint Server 2007
