While it's nice to see an open-source alternative, there has always been a very good cross-platform alternative to the 800-pound gorilla (AutoCAD) in ArchiCAD. The architectural office DiNisco Design Partnership, that has designed the last 4 schools built in North Andover, MA and whom we just hired to design another school, is an all-Mac office, using ArchiCAD. It is compatible with AutoCAD file formats and is available for OS X. I've talked to Ken DiNisco about ArchiCAD and he said that new members of the firm sometimes grouse about having to learn another CAD program (most are versed in AutoCAD) but once they do, they usually prefer ArchiCAD.
For a good list of CAD software for Mac, see:
http://www.tenlinks.com/Cad/products/MAC/cad.HTMA while ago I was reading an article about CAD software in US architectural firms and was surprised to learn that according to an AIA survey in 1999, about 33% of them use Macs. And only 53% of them or so used AutoCAD. (there is more discussion of these numbers in the article below)
http://www.architosh.com/news/2001-02/2000...jfk-story.phtml