[Note: The 2010 release of Revit has resolved this issue.]
I wanted to use a floor hosted family to create a "waffle slab". Simple idea really, just create a four void blend cluster that could easily be placed in a "thick" floor slab to result in a waffle slab. In experimenting with it I discovered that the floor volume is not altered unless the void cuts both the top and bottom planes of the floor. If the void only removes some of the floor material it does not alter the floor volume at all. Here is an image as an example.
Using Revit now for a little over six years I've managed to convince myself that it wasn't always this way...but I can't be too sure anymore as my memory isn't what I used to remember it to be. I also tried the trick of using it in an In-Place family too...no difference. Ideally a void ought to subtract volume from a floor regardless.
A second issue with this approach is that if it is used in conjunction with a floor slab created by Revit Structure that uses the metal decking profile feature...the void family will kill the display of the profile for the entire slab.
These two issues "killed" this approach for me, shame too as it would have been "easy".
A third oddity before I go, regarding the new Mass Floors feature. If you try to create a Calculated Value that uses the Floor Volume parameter Revit will reject it as an invalid parameter name. Trouble is that it is a system parameter! Here's a couple images that depict the issue.
When you examine the parameter heading value under the Formatting Tab in Schedule Properties you'll find an "extra" space at the end of the parameter name. It's my guess that this is getting Revit annoyed.
Oh all right, since I'm on a roll one more...this is hardly the only such element with this issue but you can't tag a floor with Area, Volume or Perimeter data even though the element clearly knows what they are.
Per a comment I've POSTED the file if you are interested in the "waffle" family though it doesn't really "work" for the reasons that I listed above.