In 4e user of the Divine power source have explicitly been "invested with the authority to wield... on behalf of a deity, faith, or philosophy."
That passage is from p 60 of the PHB.
Here are some other relevant passages:
Clerics and paladins call down the wrath of their gods to sear their foes with divine radiance, or they invoke their gods’ mercy to heal their allies’ wounds. (p 7 )
Divine magic comes from the gods. The gods grant power to their devotees, which clerics and paladins, for example, access through prayers and litanies. (p 54)
All clerics choose a specific faith to which they devote themselves. Usually this faith is the worship of a specific patron deity—for example, Moradin, Pelor, or Erathis. Sometimes clerics are devoted to churches that venerate groups of deities or even philosophies. (p 61)
As fervent crusaders in their chosen cause, paladins must choose a deity. (p 90)
The reference to "philosophies" as an alternative to faith in a god or gods is mentioned only twice, both times in relation to clerics. I'm not sure what to make of it. There are at least two interpretations I can think of. One is that, consistent with the other references to divine magic coming from the gods, clerical adherents of a philosophy draw their power from those gods who embody/uphold that philosophy. The second is that, in a desire to maintain some consistency with earlier editions, the 4e designers incorporated contradictory descriptions of the divine power source into the rules.
I think the second interpretation is more likely as a matter of historical conjecture. I personally prefer the first interpretation as a way of trying to make the game coherent.
Under this framework things like Secular Humanism and Atheism would qualify as 'philosophies', potentially granting use of the Divine power source. Disbelief in gods can grant the same power as belief in Pelor
I think you're right that these examples push up against the limits of "philosophies" as sources of divine power.
My problems with this approach to divine power are metaphysical and psychological. Metaphysical, what is the source of this power which is neither Arcane (whether obtained via study and practice like a wizard or swordmage, by being imbued with power like a sorcerer, by bargaining with an entity like a warlock, or by evocation of the underlying harmony of the cosmos like a bard) nor Primal (obtained from the lingering spirits and memories of one's ancestors and the world of which they are a part) nor Psionic?
If the answer is "faith" or "conviction" - and this is distinguished from the psionic power of a monk or ardent - this takes me to my second problem, which is the psychological one of what exactly this mental state is, which a cleric of secular humanism possesses, but a learned sage who remains in his library/tower despite the encroaching orcish hordes lacks?
I can work out what an initiate of a god is doing differently from others, such that s/he wields power. S/he has been initiated! But the faith/conviction/belief thing doesn't work for me, because the distribution of the
power in the gameworld fiction doesn't really seem to correspond to the distribution of any salient mental state.
Indeed, that's part of the point.

In 2e, when PS came about, there wasn't a character class who could gain power from devil-worship, but the idea that gods were relatively small (in comparison to most worlds) was driven home pretty powerfully.
OK, I can see this, but for the reasons I've tried to articulate I'm not sure it works for 4e.
Sure, Zeus might chuck lightning bolts down on your little backwater homeworld, but that's not everywhere, and it ain't here. If you worship Zeus as a planewalker, it's not because he chucks lightning bolts down, it's because there's a particular idea he embodies that you favor (forex, with Zeus, it might be the power of the masculine patriarch, expressed through chucking lightning bolts). Otherwise, perhaps it's part of your cultural or personal identity or story. You worship Zeus because he helps people where YOU come from, because he helped liberate YOUR world from the titans, because you've been personally aided by a timely lightning bolt. Zeus isn't there for all, he's there for YOU, and you have the spells to prove it!
"Magic" is belief in PS. It's not being a creator or a sustainer that gives you the power to grant Cure Light Wounds, it is (in Planescape), being the concentration of millions' of peoples' hopes and dreams, having THEM believe in you, worship you, and dedicate their lives to serving you. That nebulous quality gives real magical power to deities, which they then share back with the people via the divine power source. A god has power because people believe in them.
Just trying to state your picture back at you, so you can see if I've got it right - widespread hope/conviction vests a god or similar entity with power, which that god can then share with initiates (like clerics). So gods aren't creators/sustainers as I characterised them upthread, but an important category of patrons. (There seems to be some resemblance here to Glorantha.)
I can see how that's feasible and highly workable for a game. Right off the bat it suggests at least one sort of interesting conflict - "faithful", in this world, presumably means in the first place loyalty to the divine patron, but also - given the dependence of the divine patron on widespread conviction - loyalty to/honouring of the underlying values/hopes/commitments. Those two dimensions of faith could be fun to drive a wedge between. (The same wedge can be driven in the approach to gods I am articulating from what I take to be the 4e perspective, but I think with more difficulty - because of the tighter union beteen god and value - and therefore with different metaphysical implications.)
I still feel that it doesn't work for a game with warlocks - at least, not without something more being said about how the different categories of patronage work. I also feel that it doesn't fit with the idea that philosophies or hopes that are divorced from deities can give rise to divine casters. As you say
And via the god, that principle lets the faithful blast holy light and heal the sick. Pretty awesome.
This doesn't seem to leave room for divine power without divinity. Or, alternatively, it suggests some sort of puzzling situation where the hopes and convictions of atheists give rise to a god of the non-existence of gods, who is the source of power for atheist clerics.
(By the way, couldn't XP you for a good post.)