Alberto wrote a really fine post that should help you to better understand how many-to-many relationships can work in PowerPivot, using a lot of images that explain some concepts better than a thousand words. The example shown by Alberto is really interesting also for another reason: it can open up your mind about why Vertipaq (the engine of PowerPivot) is so important for some scenarios that today cannot be covered very well by the MOLAP engine of Analysis Services. If you think to the many complex calculations that cannot be simplified by working on aggregations and to the many complex models that cannot be correctly represented through a star-schema, you will start to understand why Vertipaq will be so useful when it will be implemented in Analysis Services for Corporate BI.
If you will attend the PASS 2010 in Seattle or the PowerPivot Workshop in Netherlands, I will be happy to share thoughts and ideas about possible future uses of Vertipaq!