If I'm a casting director, any role that I may have considered to be a role for Jack Black, it would seem that now that role is far more appropriate for Zach Galifianakis.
I don't think that Jack Black is going to disappear by I get the sense that in the world of prestige comedy, that Zach has the upper hand over Jack these days.
This weekend the comedy Due Date staring Zach Galifanakis and Robert Downey Jr. will hit theaters, and I expect that this film will be at the top of film fans must see list, regardless of whether it gets good reviews (the love of The Hangover, perhaps is setting the bar high here for director Todd Phillips).
Yet, I think that film fans will be excited to see Zach in this role, in a way that a Jack Black performance wouldn't attract.
And perhaps maybe Zach isn't a better Jack...but perhaps we'll see a line drawn in the audience the two attract. Jack Black might stick with the Kung Fu panda crowd, performing in roles like Gulliver's Travels. Where on the other hand Zach Galifanakis' comedy might skew more towards the R-rated spectrum, in films where Zach's can present the "comical fat looser" role in a way that is perhaps less, well, goofy.
And it is in this distinction, that perhaps the world will find that there is room for Jack Black and Jack Galifanakis. But if Zach had been on the scene, perhaps so previous Jack Black films (say Tropic Thunder or Shallow Hal) may have been Zach roles...maybe.