I've never heard Bettany sing and can't find any YouTube of it, but he could certainly pull off the sinister nature of Javert. And Jackman as Valjean is definitely a plus as well (even if he's not Colm Wilkinson
). If Javert and Valjean are good, that's a huge part of making this thing work.
I think Eponine might actually be the third most important role when it comes to pop-cultural awareness, yeah? Lea Michelle's big enough now from Glee and she's done Eponine before, so that should be a pretty natural decision I'd think.
On a humorous note, if anybody here is fans of the Channel Awesome crew - Nostalgia Critic, Linkara, Spoony, etc. - the Nostalgia Chick just recently did a review of the Les Mis musical where several of them break into "One Day More" at the end. And considering they're a bunch of random internet reviewers, they do a decent job at it.