While I'm not sure of the viability of directory submissions as a way of search engine marketing, or optimization for that matter, when Google disbands its products, it's news. Thanks to a heads up by Brent Csutoras, we now know the Google Directory has gone the way of Wave and Google Video; that is, it's gone.
If you try to navigate to the Google Directory, you will be greeted with the following message:
Google Directory is no longer available.
We believe that Web Search is the fastest way to find the information you need on the web.
If you prefer to browse a directory of the web, visit the Open Directory Project at dmoz.org
Again, the viability of directories in relation to marketing is dubious, but if you could get your site into one of these things -- the prominent ones, anyway -- it was worth a shot. Oh well. There's always DMOZ, although, I have something of a history with that particular subject.
Over at Csutoras' post, he states his preference for Best of the Web directory, which offers a similar level of quality as a DMOZ entry. The difference being, the BOTW directory charges for listings. DMOZ, while free, has issues of with the approval process.
Speculation concerning the closing of the Google Directory focuses on lack of use, something Barry Schwartz pointed out in a SearchEngineLand post. He also indicates Google deactivated its directory without much fanfare, and, after a quick glance at the Google Webmasters blog and the official Google blog, you'll see that there's no announcement available, which also supports that "lack of use" theory. There's also nothing on any of the various Twitter accounts for Google employees.
If no one's using a product, quietly killing it is indeed an option, although, there isn't much of anything Google can do that will remain quiet for long.