City Disc has now amended its listing to give an October 30, 2008 release, and translated accompanying text now apparently reads: "The release date for GTA 4 for PC has not yet been confirmed by the Publisher, Take 2. The release date is not guaranteed and is open to change."
(Thanks to Evo Gamer for the tip off and the translation).
As we pointed out before, it took GTA III and its sequels around six to eight months to make the leap to the mouse and keyboard crowd, so a late 2008 release for GTA IV on PC would be in keeping with tradition. But it doesn't officially exist. Yet.