Project Spark looks like a striped down version of Revit and does not support conceptual design, analysis, rendering, advanced collaboration, and API. It may turn out to be a Revit LT version of sorts. I would guess they would come out with all flavors of Revit, but they are only testing the architectural version. MEP and Structural hopefully would be available when released.
Currently, you cannot open Revit Architecture or Revit MEP 2012 in Project Spark. Files created in Revit are not supported in Project Spark. However, Revit 2012 files can be linked into Project Spark.
You also cannot open Project Spark files in Revit Architecture or in Revit MEP 2012. Because Project Spark uses a newer Revit file format, it is not currently possible to open files created in Project Spark with Revit 2012.
They're looking for feedback on the overall concept, usability, and learnability of the product.
You can download the preview here.
You can view the tutorial here