Thanks to Ringer Gaer Carreg for doing some digging and sending in this Oct 6th exchange between the official twitter accounts of actor John Bell and Luke Evans.:

@johnbell123 John Bell 
@Mr_Luke_Evans thanks Da! Happy now! See you soon. X
6 Oct 

Luke Evans
@johnbell123 no problem son! See you soon back in NZ
6 Oct

Of course nothing is official until Peter Jackson updates us on his Facebook, but I think its safe to call this one: John Bell will play Bain, Son of Bard. [View Exchange] [Full Cast Listing] [Official John Bell Twitter] [Official Luke Evans Twitter] The New York Times is also listing John as Bain in their official cast listing here.