I have consulted with my legal counsel (my wife) about this issue. The policy to which Kelemvor referred specifically applies to fourth edition materials ("...is limited to use solely to refer to 4E and the Core Rulebooks"). Remember that, depending on the date of publishing, the source to which you are referring could be the intellectual property of TSR, WOTC, Paizo, the venerable Sage himself, or another loremaster (Steven Schend, Dave Arneson, etc.).
I always cite my source and, at least, give a layman's nod to the creator in the form of a disclaimer of my own making. This generally suffices because no one here is making money from this site; it is a game run by fans for the sake of fans, much as any game you played at home. It's a lot like if your buddy Bob had a copy of the Player's Handbook and read you something out of it. That being said, it always pays to check to see if there is any specific guidance for the material you mean to cite and, if there is, adhere to that guidance.
No wants to put our very generous game designer in an actionable position, or steal work from the creative men and women who have framed our imaginations in their ideas. The staff will censure any material believed to hazard that. Thanks!
"A man may die yet still endure if his work enters the greater work, for time is carried upon a current of forgotten deeds, and events of great moment are but the culmination of a single carefully placed thought." - Chime of Eons