The way it has worked in the past is that a few, usually, nationals and 1 or 2 other events were broadcast on nasa tv. this means that anyone with the nasa tv channel can watch it. It also means that you can get a webcast very easily. Team 188 keeps a very good list of places that webcast nasa tv.
http://www.team188.com/2002/videos/nasatv.shtml All that this announcement is about is the NASA tv broadcasts, and it is much better then before. These are the much more expensive ones to do.
NASA has also webcast many other regionals. This was done seperately from the NASA tv (as far as I know). Information about those would be posted on robotics.nasa.gov. As of right now, nothing has been announced.
Remember that NASA, like everyone else, is having funding trouble. Don't be suprised if your favorite event is not webcast. But, don't give up hope yet, either.