http://www.homelandsecurityus.com/FAQ.htm That is a link to their FAQ page which says who they are, how long they have been doing this, how they get their information etc. There are many questions and answers there.
As far as these threats go, I don't think any of us know for sure what will happen and that could also include our Homeland Security Department. I do feel they are using the internet for communication because it's the only thing they have left where they can hide their location (if they are smart enough). So, it's possible some of the messages we've been able to track down and post here are legitimate. It's also possible some are just postings by whackos with nothing better to do. Bottom line, we will probably only know what is true when something happens unfortunately.
Hope that helps.
Bottom line, we will probably only know what is true when something happens unfortunately. "Prediction is difficult, especially when it concerns the future" -- Lazarus Long, Internet personality circa 1982