I like the idea of Renee Zellwegger also, db - if she would gain back a few pounds. (I don't like the stick figure Renee of the last year or so.) I thought of Helena Bonham Carter for Mina just because I think she has the right look and could do a refined upper class American accent well, I'm sure.
I would love to see Jake Gyllenhaal do Finch. I've only seen him in Brokeback Mountain, so I don't have any other preconceived notions about him, but I think he'd be a fantastic Finch.
And I'm glad someone else (dharma_bum?) thought of Johnny for Walter. I love Walter and I think - even if it was only a voiceover part - Johnny could do so much with him. And who knows what other opportunities we'd get to "see" Walter?
Those were the only three that I came up with as I read the book, but I like some of your other suggestions as well.
Live! Life is a banquet and most poor suckers are starving to death! ~Auntie Mame