For those interested, Sibley did indeed create a plate for the Ivory-billed Woodpecker, but included the species in the 'extinct' section instead of including the plate in his The Sibley Guide to Birds. The guide was published before new evidence was found showing perhaps the rarest species of bird in the world flying in the Florida Panhandle in 2004. This was the first 'verified' (there's still debate about Cornell's reports) sighting in 60 years.
You can see the video footage here at YouTube, or check out some compelling new evidence here.
It's up to the viewer. Although some eyewitnesses are not birders, I believe there are at least a few birds probably still persisting in the south that have been seen by locals there. Mind you, the species is obviously in extreme threat of extinction and no extensive searches have produced definitive evidence so the outlook is grim.
This news story has also seen a few books published on the events, including The Grail Bird: The Rediscovery of the Ivory-billed Woodpecker by Tim Gallagher and In Search of the Ivory-billed Woodpecker by Jerome A. Jackson.
Finally, as promised, here is the plate that is missing from the guide. You will need Adobe Acrobat to open the file but it's worth it. Certainly this elusive bird will continue to mystify birders for a long time.