I think one of the problems with WotC releasing a Greyhawk campaign book is that it'd possibly be more fluff than crunch. With FR, there's a bit more variety & whatnot available in the campaign, as well as unique setting elements that need to be included in the campaign.
However, with Greyhawk, there's not really any of that. It's intended to be the default setting for the core books (and Dungeon modules now, IIRC), so I don't think it'd work introducing new setting-specific systems into the game (like Spellfire for FR, Dragonmarks for Eberron, etc.).
For all we know, the Living Greyhawk Gazetteer released during the 3.0 days may have been the "testing the waters" product, and sales may not have indicated a good result. Granted, IMHO, the Gazetteer wasn't really a page-turner--it too was a lot of fluff with little crunch, but it may have feasibly sealed Greyhawk's revamp fate. Add to that the buggy nightmare that was the Temple of Elemental Evil CRPG, & Greyhawk's got a bad appeal for the potential market of new players.
Granted, I think there may be the slim chance that a Greyhawk game could work well, but I'd think it'd need a few things: a nice world map (perhaps copies of the 4-piece Greyhawk map included in Dungeon, or a 1-piece version of that map); a good map of the Free City of Greyhawk (since it's what the setting's name is derived from); details on the folk of Greyhawk (and possibly regional feats/benefits, from Dragon magazine); Greyhawk-specific crunch for PCs (reprinting PrCs like the Radiant Servant of Pelor, Suel Arcanomach, Shining Blade of Heironymous, Temple Raider of Olidammara, etc.); notable monsters (Suel Lich, Xvart, Greyhawk Dragon, etc.); & notable NPCs (a reprint of Warduke, Mordenkainen & the Circle of Eight, Iuz, etc.).
Now, the release of Expedition to Castle Greyhawk shows some hope for the setting. However, we'll just have to wait & see how things go.
