In Revit 2011, you can cut one solid with another solid. I have included additional information below:
This functionality is limited to the conceptual mass environment or when creating a single in-place component mass.
It works within the limits stated by Ryan in his reply: in the conceptual mass environment or in a single In-place mass.
You can also do this by setting the material properties as the same and joining them together, once you do this you can hide the element you don't want in view or place it on a workset like 'invisible' and uncheck that in the VG interface! I do this all the time to get the desired profile on Curved walls that need a sloped bottom for instance.
I hope this helps!