Have you changed location (e.g., in a different country)?
See this article about countries that support Siri: http://www.apple.com/ios/feature-availability/
Also, from the HT4992 article you attached: "* Siri is currently in Beta. Siri requires Internet access. Siri may not be available in all languages or in all areas, and features may vary by area. Cellular data charges may apply."