Well, it's not particularly sensational news so they have no reason to make this up. It hasn't come up on many news sites, but none of the new cast members are A-list stars, so that's probably why.
On that list the only names I know are Irfan Khan and Konkona Sen Sharma. She's a young actress and has received a lot of critical acclaim. She seems to do mostly less commercial projects, but has received a lot of attention despite that. I've only seen her in a small part in "Omkara", but I really liked her there. She definitely makes a lot of sense for this film.
I wonder if this means Bipasha Basu is out? Surely the only significant part for a young Indian lady is Kavita? Unless they plan on casting her as Prabaker's mother, but she's a bit young for that I think... (although I could imagine her doing an excellent job with that part - perhaps more than I can imagine her doing with Kavita)
I checked up Prabhu Deva on imdb, I think he does more choreography than acting and it seems a lot of his acting filmography consists of guest appearances as a dancer. So I wonder if that's how they're going to use him in Shantaram? Will he be doing the choreography as well?
The only film I know on Prabhu Deva's filmography (choreo not acting) is "Bombay (1995)" which is about the Bombay riots of 1992. It's told through the eyes of a mixed marriage (Hindu husband, Muslim wife). The kind of choreo and dancing that is in the film makes a lot of sense in Shantaram context. It has a rather more traditional feel to it than a mainstream Bollywood film would usually have (the protagonists move to Bombay from a small village). You can check out one of the dance numbers from the film to get a feel for it:
http://uk.youtube.com/watch?v=ctYrcsM7FXUThe other actors seem to do mostly supporting roles from what I can gather, some of them seem to do stuff in Kollywood (commercial Tamil language industry) rather than in Bollywood (commercial Hindi language industry). In fact Prabhu Deva's Bombay is more of a Tamil film than a Hindi one (though it mixes both languages).
So basically it seems like Mira is going after Indian actors and actresses who do good supporting parts and interesting art house projects, rather than the A-list stars (with the exception of Mr Bachchan of course). I wonder if that might have anything to do with the decrease in budget (personally I was expecting a few more A-list stars and the previous mention of Bipasha Basu for Kavita made a lot of sense as she's a far more commercial actress). The A-list Bollywood stars don't earn as much as the biggest Hollywood stars of course, but $1mln per film is not unusual AFAIK and I guess that's not the kind of money Warner Bros would spend lightly at this point.