Echoing some of the other sentiment here, my personal feeling is that if I make available a project where I have used someone else's code, I will try to credit any other authors as best I can. Usually, that's as simple as leaving in their comments and readme information. Sometimes that involves a link to some other site (like to a thread in this forum where someone posted something I used). To my way of thinking, that's a courtesy I would appreciate and I would do my best to reciprocate. That said, when I post something that someone else uses in his project, I understand that any acknowledgement is just a courtesy and not something I could require, even if I wanted to.
However, my understanding is that, from a legal standpoint, the material posted on IGN's NWN Vault can be republished without permission or acknowledgement of the earlier authors. It's not that any authors have given up their rights, it's that they have given others the same rights to publish the work when they made it available on the vault.
[...]You hereby grant, transfer and assign to Ziff Davis and its successors, assigns and licensees (collectively, “Licenseeâ€) a fully-paid, royalty-free, irrevocable, perpetual, worldwide right and license to publish, distribute, reproduce, transmit, use, translate, display, perform, modify, revise, create derivative works of and archive the Material, in any form or media now known or hereafter developed (including without limitation in print, magnetic or electronic form), on any number of occasions in any form, and to sublicense third parties (including other users of the Service) to do any of the foregoing with further right of sublicense[...]
Which, with the slightly more relevant parts highlighted, reads something like
[...]You hereby grant, transfer and assign to Ziff Davis and its successors, assigns and licensees (collectively, “Licenseeâ€) a fully-paid, royalty-free, irrevocable, perpetual, worldwide right and license to publish, distribute, reproduce, transmit, use, translate, display, perform, modify, revise, create derivative works of and archive the Material, in any form or media now known or hereafter developed (including without limitation in print, magnetic or electronic form), on any number of occasions in any form, and to sublicense third parties (including other users of the Service) to do any of the foregoing with further right of sublicense[...]
In other words, ZD and other users of the service (which is basically everyone who used IGN's Vault) can archive, modify, build on, etc. and republish the stuff that was posted on the Vault. That's partly why a different part of the agreement requires that the poster actually have permission to publish what he is posting; he can't just post any material because it will be republished.
BTW, I once asked Rolo about this (for a related matter) and his response was that, in essence, that provision (or one like) it is what allowed the creation of the new vault. That is, if the individuals who posted their stuff to the old Vault hadn't agreed to the old Vault's agreement, there would have been no way to create the new Vault.
Obviously, this isn't an arrangement with which everyone would be comfortable. People who want to allow more restricted access to their work would have to find a different way to make the material available. And, of course, deal with the difficulty of enforcement of any restrictions, which, IMO, as a practical matter, is a far more onerous task.