I am down in South Carolina with Austin and Chris, working on integration issues with the new ATC system for X-Plane 10. One issue Chris is working on now: how do you fit the wide open design options of Plane-Maker into the narrow FAA categories for aircraft. If someone builds a rocket powered lighter-than-air piano with a tilt-rotor and a lift fan, how many miles of separation does that aircraft need from the 747 in front of it? What SRS category is Sponge-Bob Square-Pants, or a Steinway piano, or Snoopy's dog house? (And don't even get me started on some of the weird creations Propsman has sent me!)
Our current solution is to analyze the structure of the plane and use the most conservative criteria available. If your aircraft weighs a lot, it's a heavy, no matter how you built it. If it has jet engines, it's a jet, etc. This way airplanes modeled after real life will get correct categorization, and custom designs will at least receive some kind of vaguely sane handling.