I hope that they explore Darwin's personal struggles and belief more deeply than the plot description suggests. To sell him as an athiest smacks of wanting to stir up controversy -- Darwin always called himself an agonostic, and his rejection of Christianity stemmed as much from the tragic death of his daughter, Annie, as his research. It's a complex and fascinating biography, one that, as a lover of science and the history of it, I would love to see done with real justice.
The cast is certainly more than capable -- Bettany and Connelly are both wonderful, and they'll be joined by Jeremy Northam, Toby Jones, and Benedict Cumberbatch. Annie Darwin has yet to be cast. (I can't help but be amused by the fact that Bettany and Northam have played Darwin-inspired characters before, in Master and Commander and Possession, respectively. Pet interest, perhaps?)
I know the merest mention of Charles Darwin is enough to cause an epic flame war in the comments. If it's possible, try to keep discussion only to the film, the actors, the book, and Darwin's biography. That may be too much to hope for, but I beg you to remain respectful and tolerant of one another.