I've heard that if one has an IMDB Pro account, it's fairly easy to add information, ie., it is "believed". I still take all of these tidbits as rumors for now. Sometimes they do end up being correct. For what it's worth, when Sweeney Todd was in pre-production, we started hearing little ditzels of info from various unexpected places, such as the school that Jamie Campbell Bower and Ed Sanders attended (congratulating them on snagging their roles, of course!) And I think that we may have heard about Laura Michele Kelly from her record label site or something. And then gradually, things were confirmed. Since filming is still several months away I don't think it's at all unusual that we haven't heard anything specific about casting- I hope to start hearing a few names after the New Year.
Edited to add- it just occurred to me that Sweeney Todd is a special case because they actually did all their singing the month or two before filming began. So the roles may have had to be confirmed a little earlier. By the way, even though Johnny was heavily rumored to have a role in Alice In Wonderland (for the obvious reason that he's Tim's main muse), I don't believe that was confirmed until after filming had already started!
Cathy