You see, this is why most game companies don't allow their developers to even look at fan ideas, and use community managers to filter input from forums and the like. There are just too many legal issues tied up in using someone else's ideas, with or without intent. Unless, of course, the content is published to public domain, as I've done once or twice...
Of course, Steam's terms of use explicitly state that by posting something on the forums, "you expressly grant Valve the complete and irrevocable right to use, reproduce, modify, create derivative works from, distribute, transmit, broadcast, and otherwise communicate, and publicly display and perform the User Generated Information and derivative works thereof in any form, anywhere, with or without attribution to you, and without any notice or compensation to you of any kind." Which basically means he's SOL for getting any credit to, control of, or compensation for the idea.
