I am one of many many people who only wishes that AutoCad was available for Mac. Too bad it hasn't been since release 12, and while there was rumour that it was coming back to Mac about 2 years ago, this seems to have all but been forgotten.
The Virtual PC route is plausible, but not entirely bug free (the cursor doesn't update properly to reflect the choosen tool) and it is not fast enough to reasonably handle 3D (not that 90 percent of ACAD users ever go into 3D).
VectorWorks is a better possibility, but is really geared to the architectural side of things, as is ArchiCad. If you are looking for a modeler with limited draft capacity you could try FormZ (which is now in version 5)
Does any one know if Bentley MicroStation runs on UNIX. If so that could be a possibility as well.