Whoever gossips to you will gossip of you - Spanish proverb
Whilst the rumours are not confirmed, the evidence, summarised here, appears fairly compelling.
Pulling apart some of the game's textures reveals stunning similarities with, for example, water textures from Half-Life 2 and lighting textures from Doom 3. In the case of Doom, some of the texture filenames were even the same between the two games, with "lights_impflash.dds" present in STALKER, and referring to the Doom Imp monster.
There are many theories as to how this could be the case - STALKER's development was notoriously piecemeal, with much of the original team replaced at various points through the project. These could be placeholder textures put in by one team and not recognised by another. But the idea that permission had been gained was put to bed by Id Software's Todd Hollenshead, who told Shacknews that "From what I've seen on the web, it's concerning."
There's yet to be any word from GSC Game World, the developer, or THQ, the publisher. Expect this to kick off early next week. ยต