Knightley has been taking singing lessons ever since the possibility came up (and I actually think she proved she had a good voice in The Edge of Love), and I think she'll be absolutely charming as Doolittle. But then I'm biased towards her. Even if you aren't (and I expect many "too skinny!" comments), I think the combination of Wright and Thompson pushes this into very, very promising territory. Thompson can do no wrong by me.
What will really tip the balance is who they cast as the grumpy, misogynist Professor Henry Higgins. The Telegraph reports that Daniel Craig is being considered, and he'd certainly be ideal as the grim Higgins, probably moreso than the dapper Hugh Jackman. I wouldn't mind seeing Patrick Wilson become a contender, and I half wonder if Gerard Butler's vocal chords were trotted out on Saturday Night Live as an audition. Just you wait, and practice your R's, and we'll see whose face Knightley must become accustomed to.
UPDATE: Screenrush caught up with Joe Wright, who says he never signed on, and is uninterested. Given the shakiness of the initial report, it's unclear whether Knightley's casting is official either.