Brought here to keep it from horribly derailing someone else's thread. :^)
A point tangential to a question made in another thread was that some of the 2e Planescape material -- Guide to the Nine Hells, IIRC -- explicitly said atheists (again, explicitly mentioning Athar) end up in the bottommost layer of Baator, there to be devoured by Asmodeus. The Forgotten Realms campaign setting apparently suggests that those who worshiped no god also get trapped in the plane of judgment and eventually face oblivion, though I'm quoting secondhand on that.
Personally, I refuse to countenance any rule that automatically sends somebody to Hell (or oblivion) regardless of alignment. This thread, then, is for talking about where you'd think devoted atheists go. They don't only have to be Athar -- some could be from Prime worlds where the gods are are very weak, manifesting rarely. It could also be a concern in Urban Planescape. It's been a while since I read about Athas, but don't the clerics worship the elements there? Are there afterlife realms on the Inner Planes where fellowships of Athasian believers' spirits congregate after death, to spend time in fellowship meditating upon the nature of reality?
For one example, it would amuse me if the power of the Athars' beliefs had erected for them an afterlife on the Astral -- a mobile community of spirits searching the Astral eternally for the Great Unknown, with the spirits occasionally becoming one with the act of the searching itself (and perhaps finding the Great Unknown in the process).
Perhaps, like any other believer in Planescape that's not sincerely tied to a specific god, they simply end up in a realm which matches their alignment. However, because they do more than simply not care about gods, but have a positive attachment to atheism that is of the flavor of a given alignment, they tend to end up in areas very far from powers' realms.
Thus, maybe the lawful evil atheists do get nabbed by Asmodeus, instead of ending up as spirits that might get noticed by lesser LE gods. On the other hand, you could have lawful good atheists, who followed a code of morals because they believed it right without requiring the imprimatur of a deity, go directly to Chronias, out of reach of powers known to mortal ken! Maybe chaotic good atheists find contentment in the sands of Pelion, and chaotic evil atheists have their own, personal layer of the Abyss to welcome them.
Stephen Hawking, asked if he believed in God, replied, "Yes... if by God you mean the laws of the Universe." Plenty of atheists might give the same response -- and be counted LN, finding beauty in truth and believing it important that we accept what is, without wishful thinking. To become part of the machinery of the Universe they had studied for so long might appeal to some academes. The same would apply to those atheists who, like Xaositects, who postively affirm a lack of divine plan in existence, and take comfort in this: they consider it important to take each moment of this life as it comes, and then end up in Limbo (where everywhere is as far away from anywhere else as anywhere else).
The utter nihilists, more maltheists than atheists, who truly believe the Universe has cursed them with death and curse it back, wreaking destruction just to throw a temper tantrum on the way to oblivion, or who despair before the emptiness of it all, find themselves not before Asmodeus, but instead embedded in the monoliths of the Grey Waste. Those who simply focused on living their own lives following their day-to-day interests, true neutrals, wind up in the Hinterlands of the Outlands, and those who truly tried to help others, trying to bring about a better multiverse without any expectation of reward in an afterlife, find themselves shining quietly in the firmament of Elysium over Thalassa.
And finally, there are those atheists who reject gods, good, evil, law, chaos, and everything normal in inexplicable and affirmative rejection of, very nearly, existence itself. If one hewed to the very politest, least offensive interpretation of "atheist," it might have been these souls damned to oblivion in 2e... but we've got a much better place for those souls in 3e. A far, far realm indeed...
[moderator]As a note: Given the potentially sensitive nature of this topic, I would like all participants in this thread to keep in mind that we're all friends here. Keep it above the waist and civil so's I don't have to use my 'mommy voice'. [/moderator]