Fears that terrorists would quickly find needed information on the Internet have proved, so far, to be unfounded. Its not because the information isnt still there, it is. Check out http://eyeball-series.org/.
And all those government sites that were shut off from public access after September 11, 2001. You can still get the information. OK, not the new stuff, But the original material is still available via the cache links on Goggle, or by using archival sites like http://web.archive.org/web/*/www.strategypage.com (which, in this case, shows what strategypage.com looked like through the years, via software that literally downloads most of the site intervals.) The web archive site captures most major web sites often enough that little is lost when the site is taken down.
There are other, more technical tricks, for getting at web information that has been removed from public view, but these are not as reliable, or as legal, as the ones described above. There is a real risk that terrorists could use information on the Internet to plan attacks. But many of the vulnerabilities are only useful to well trained and well prepared attackers. Fortunately, few, or no, terrorist groups have the technical experts to exploit web based data on utilities, industrial plants and other targets. But the potential is there.