"The new conceptual modeling environment, however, is a mixed bag. On the one hand, it lacks the fluidity and ease of use of tools such as SketchUp and the new Bonzai, and I don’t see it replacing the ubiquitous use of SketchUp in architectural design, given the fact that there is a free version of SketchUp readily available. On the other hand, Revit’s conceptual modeling environment is much more powerful and sophisticated. The heads-up dimension display that shows the dimensions as you are modeling and allows them to be edited—which is so helpful in the detailed modeling stage—also allows conceptual masses to be created to the exact level of accuracy desired. The ability to tie geometry to reference lines and planes, and add constraints and parameters makes it easy to iterate through many design variations easily and quickly. The new surface dividing, patterning, and panelizing capabilities makes Revit’s conceptual modeling environment even more compelling. The ultimate, of course, is the Building Maker capability, which allows the massing model to be converted to actual building elements so that you don’t have to start over to create the BIM model."
Lachmi Khemlani just posted a pretty comprehensive review of Revit 2010. There's a section on API tools that reference some models that you can read more about here. Here's a snippet about the Conceptual Modeling tools: