QUOTE (wind_in_the_stones @ Nov 16 2009, 10:59 PM)

Nope. My memory is bad, but I don't think it's so bad that I've forgotten posting about it. Nor do I remember even wondering about it before. Nor do I recall ever seeing someone else's version of the question, but I could be wrong about that. And I haven't been reading here lately.
I see that the rule says either/or, but the problem is that I just don't understand how it's possible. We know that it is possible for the mage to target any of the people in question with a mana spell. We also know that (SR4 p173) Area spells affect all valid targets within the radius off effect, friend and foe alike. Neraph, you didn't include the entire sentence in your quote. It says "An astrally perceiving mage can cast spells at a target in either the physical world or the astral plane." (emphasis added) So the wording is slanted towards single-target spells. Also, it specifically says "target", and we don't (necessarily) target people with AoE spells. For example, I choose to target that spirit on the astral plane (or that point in space), and those two people on the physical plane happen to be in range, and get blasted too.
it's quite simple, really.
you are astrally projecting. you drop an AOE spell on a magician's spirit. does the AOE spell hit everything on both planes? no. no it doesn't. because it can't cross the planar barrier. you can certainly see the other people. but that doesn't mean you get to hit them. why? because the spell cannot cross the planar barrier. likewise, when you cast a spell on one plane, even if you yourself exist (in some form or another) on both planes via the dual natured power or astral perception, the spell still cannot hit both planes, because there is a barrier in between the planes.
the closest thing i can find to a quote supporting the astral/material barrier is
QUOTE (SR4A @ p 176)
When the signal is received,
it channels mana through the target to create a
specified effect (thus Direct Combat spells bypass armor,
because they affect the target from within). All of this
occurs on the same plane—physical or astral—as the
magician and the target.
Area-effect spells work roughly the same way...
now, this may not be the strongest endorsement, but i am not aware of anything that particularly indicates that it works otherwise.
in any event, if you're just looking for a way to screw someone over and you're not willing to accept that your viewpoint may be wrong, you've come to the wrong place with the wrong question.