I've wondered what form Brandom's book would take. Many works on Hegel's Phenomenology are fairly straight forward commentaries. H. S. Harris's Hegel's Ladder goes far beyond any of the many commentaries in terms of its detail and comprehensiveness. Pinkard's is an interesting Sellarsian take (with some serious Barndomian influences). But I could not imagine Brandom taking the time or interest in this kind of scholarly and reconstructive work. Now after seeing what he's done with Kant and Hegel in his Woodbridge Lectures, it became clearer there was no chance of this. But how does one write on Hegel's Phenomenology of Spirit without getting caught in commentary mode. One option is the general Heideggerian approach: by writing about a historical figure you write a commentary on yourself. Well, after a brief perusal through Chapter 8 of Brandom's A Spirit of Trust, the title of what apparently is the long-awaited Hegel book, it seems to be somewhere between the traditional commentary and the Heideggerian approach (though this is a fairly speculative comment). There are lots of long quotations interpreted through Brandom's philosophical framework.
The chapter is itself long (256 pages in Word), so, as SOH-Dan points out, this will likely rival Maxing it Explicit in size, but I wonder to what extent it will influence how people understand Hegel. My bet is that Brandom's own philosophical work on inferentialism, semantics and normativity will have a greater influence on Idealism studies than his own commentaries on Hegel or Kant. There is some historical precedence for this. Look at the influence Sellars has had on Kant studies or even McDowell. Strawson's work on Kant might be an exception but the debate over transcendental arguments, one of his greatest legacies, stems originally from Individuals and not the Bounds of Sense. But maybe I'm overstating things in the case of Strawson, he did after all make it permissible, along with Bennett, for Anglo-American philosophers to take Kant seriously. Anyway, these are just some cursory half-thoughts.
You should checkout Brandom's very funny "Untimely Review of Hegel's Phenomenology of Spirit." Brandom's so-called review can be found here on his webpage, and his chapter along with other Hegel papers here.