I don't think her voice is *that deep. It seems to me to be about the same pitch as fellow countrywoman Nicole Kidman (who's also over six feet tall ;)). Either way, Hollywood seems to love* gals with British/Irish/Australian/New Zealand accents, so I think we'll continue to see more of her.
For what it's worth, I still think Julia Stiles has the deepest (and most monotonous :P) voice of all actresses out there.