While news of Nihilistic's exit comes just a day after Blizzard announced the game is (once again) delayed, a Blizzard representative clarified that the two announcements are not directly related. It appears that Nihilistic is under contract to begin work on a new project, and have completed their contribution to Ghost. The Blizzard representative explained that Blizzard is considering other developers to assist in completing the game, which has now been pushed back beyond 2004.
By all accounts, Starcraft: Ghost's development is proceeding as otherwise planned (and, admittedly, delayed), by Blizzard. However, given the company's reputation for incredibly polished, high-quality products, we have little doubt the extra development time will be worthwhile.
More news as it arrives.