For Jon Irenicus it just "has" to be the man behind the voice, David Warner. All through the game I keep thinking of Irenicus as looking like the character "Evil" that Warner played in the movie "Time Bandits". LOTS of similarities. Ed Harris wouldn't be a bad choice either though.
How about the actress/model Iman (David Bowie's wife) as Viconia, or perhaps Tyra Banks?
Johnny Depp would be a great Haer'Dalis.
Tim Curry as Edwin LOL
He's a great choice if you want Edwin played with humor. Hey, he's even had the experience needed to do the "Edwina" change with his role from Rocky Horror.
Just on looks, Colin Ferrell would definitely fit and be great for a more serious Edwin.
Put a beard on George Clooney and I think you'd have a good Keldorn.
Jessica Alba would be much better as Imoen than Viconia IMHO.
Couple I haven't seen mentioned yet:
One character we seem to all have missed (avoided?) is Anomen. Argh, tough call, maybe Ioan Gruffadd?
I'm stumped so far for Mazzie?!?!
Just checked out the collage, Indeed a great job Iuz!!!