Although the 'Default.aspx' bug still stands, I have an acceptable workaround for this issue that will get the home page and site navigation working again.
Copy the existing 'Default.aspx' page from the site virtual root to, for example, 'Index.aspx'. Then, assuming you've called your home page 'Index.aspx' in Sitefinity, you can configure IIS to serve 'Index.aspx' as the 'Default Content Page' for your virtual directory. Any links you had to the home page should now be updated to simply point at the site's virutal root, e.g., '~/' instead of '~/Default.aspx'. In short, rely on IIS rather than Sitefinity to redirect to the correct page.
I must point out, though, that a direct browsing of http://your.site.com/Default.aspx
will still result in the rendering of the 'Under Construction' page I cited earlier -- even if you add '~/Default.aspx' as an 'Addtional URL' of the 'Index.aspx' page.