Information on this model and details on how to implement it can be found on the Northwest Cadence blog site -blog.nwcadence.com. Additional information will be posted from time to time on this site.
Here are the first two links to specific articles (I'll update this with additional links as they get posted):
In a nutshell, this model was created because I needed to be able to create more detailed metrics without all of the hassle involved in using the warehouse adapters that are required for being able to update the standard TFS Cube. In addition, I needed to meld information from different sources (for example, one implementation that I've created allows you to pull data from the Tfs_Configuration database and integrate it with data from the collection databases). PowerPivot and its big brother the Tabular model are well suited to these types of needs.
There are some drawbacks to this approach, but they are discussed in the initial blog post related to this project.
Please put suggestions in the comments and I will review and update as appropriate - or provide guidelines on how to update the model.