I’m not sure how many readers have been playing around the massing in Revit 2010. One of the big items to be aware of at the moment is; when you join geometry within the mass, you lose some of the mass element editing tools. After you join geometry you cannot use the “Add Edge” or “Add Profile” tool on the element. In order to add an edge or profile, you need to first un-join the two elements. You can then add an edge or profile. When you are finished you can then join the objects together again.
I have a support request sent off to Autodesk on this one. If this is intended functionality it’s not documented very well.