See: [blog.netscape.com...] (as reported on Slashdot)
The RSS 0.91 DTD was being accessed 4 million times a month and AOL/Netscape have simply decided they aren't interested in RSS 0.91 any more and don't want to pick up the tab for hosting the DTD.
The outcome of the DTD deletion is that the thousands of RSS feeds using Netscape's RSS 0.91 will become automatically invalid from July 1st 2007 (including, incidentally, the WebmasterWorld RSS feed). Most RSS readers do not need to access the DTD, but for the few that do it is important to have a plan which covers the DTD deletion. You have a couple of options. If you aren't using the entity references defined in the Netscape DTD, then you can switch your feed to Userland RSS 0.91 or (better) update to RSS 2.0 or Atom. If you absolutely rely on the entity references in your feed content, then you should copy the DTD locally, and change the doctype of your feed to reference your locally-hosted DTD.