What's the difference between magical and non-magical poisons? Are all the poisons listed in the DMG non-magical? If you use the poison spell the effect is magical? How about once someone fails the save - it's instantaneous so is it thereafter non-magical?
I don't think there's anything in the rules to make a distinction between the two. The poison entry just alludes to a difference between Ex. and Su. abilities, but that's probably for things like antimagic fields.
Whether WotC considers that there are extant magical poisons is a big part of the question. The list of diseases in 3.5 now includes magical diseases. The list of poisons does not. I've never seen a WotC splat book (not counting random d20 stuff) offer a magical poison that can be purchased (but I haven't read them all). So that leaves us with determining whether any given spell qualifies as a magical poison.
Opinions in this very thread seem to differ as to whether there are magical poisons. I think SoS says they exist:
The SRD does have magical poisons, though.
and Greenfield says they don't.
Druids of the appropriate level are immune to all poisons. They might have included similar phrasing to emphasize that this included even magical or supernatural poisons, if there was such a thing.
<shrug>
Which is it? The WotC doesn't help us figure it out because it says Spell and Spell-like poisons are "possible." Talk about a nebulous statement in the context of a spell called "Poison." I don't know how you could consider the poison created by the Poison spell, non-magical, but then maybe Create Food and Water is similar. Do food and water created by the spell work inside an AM field? If the answer is yes, then one could certainly argue that poisons created by spells are themselves, non-magical at the time the person is required to save. This would make any creature with simple "immunity to poison" immune to it, but not settle the ultimate question.
How about once someone fails the save - it's instantaneous so is it thereafter non-magical?
I believe I asked this question to SoS earlier. If you cast Dispel Magic after someone fails their saving throw, does that cancel the poison and the need to make a secondary save? He did not respond.
The poison entry just alludes to a difference between Ex. and Su. abilities, but that's probably for things like antimagic fields
I believe that poisons listed as Ex are considered "poisonous effects," though I could be wrong. While this distinction does affect anti-magic fields, I believe it's origin is rooted in the philosophy that the poisons are created naturally by the creature (with Black Dragons in Dragon Magic being able to convert arcane spells to poison attacks as the only exception I've seen). This also comports with WotC's statement that "almost all" poisonous effects are Ex.
The bottom line is that WotC does a heck of a job of dancing around the question of whether there really are magical poisons without ever stepping directly on it.
EDIT:
"poisonous effect" may also be an attempt to refer to the effect of the poison, like the ability loss or secondary save. I'm not 100% clear as to what they are truly trying to distinguish when they use that term.