A friend of mine has been making 2D floorplan tools in Flash since MX.
A couple of years ago I built a 3D option for him, based on the then open source Flux player(X3D).
In the meantime, maybe flash has come of age to do this type of thing?
I've seen Autodesk's attempt, and while I understand their thinking, for me going totally isometric/single angle is just too big a compromise.
As a proof of concept I used a couple of hours making a demo in Away3DLite. Out of laziness and lack of time I've used the Plane primitive for walls, where a custom class is called for to add thickness and get around z-ordering issues.
So here it is, the simplest 3D editor you're likely to see :)
Use the pencil to draw a few boxes and hit the "3D" icon for a rough 3d model. Be warned at the moment the eraser just clears everything, without a prompt.
UPDATE: Save floorplan or 3d drawing as .png, straight to your harddisk.
Flash 10 required, but hey, you wouldn't be here if you didn't have FP10, right? :)