I just found Sweet Home 3d - an open source 3d interior design application.
Though it`s not really intended for general purpose architecture, i think its quite worth discovering. Maybe it could be a starting point for real cross-platform open-source BIM ? SH3d already has parametric objects, it can import .3ds and .obj.
Yorik, can java app theoretically be integrated in freecad?
Together with Processing http://processing.org/
it could rock`n`roll in the future.
Or is it just a sweet dream?