I don't think the book can be perfectly adapted to a movie (the style of the writing, footnotes, rich background on historic magical texts, etc. were a large part of its charm).
However, if they could just pull off the rousing magical fantasy adventure bits in Victorian England setting it would be good enough for me.
Johnathan Strange - Johnny Depp
Mr. Norrell - Anthony Hopkins?
Stephen - Denzel Washington or maybe the guy that plays on 24
Mrs. Strange - Keira Knightley (just because she's hot)
Lady whats-her-name = Rachel Weisz (ditto)
Childermass - Gary Oldman or Alan Rickman
I realize I'm probably overbudget at this point...