I'm an OS X user, so I too am interested in getting Safari working. On Apple's webdev mailing list, they promised they'd publish the changes to HTMLarea that they made to get it to work. So, Kupu is just waiting for some docs.
The Silva version might also use Sarissa for some server interaction. Sarissa has had support for Safari 1.2's XML capabilities. Plone, though, used client-side XSLT in the drawers, and Sarissa doesn't yet support Safari's XSLT. (It isn't yet clear to me if Safari 1.3's XSLT engine has a JS interface.)