When I read a power description the Target entry comes before any Hit, Effect entry, so the first thing that comes to my mind is, targets are chosen/selected before the power goes off - but in your example the Target entry is "Target: Each creature in burst", so there must be that Burst, before the targets can be determined. That's how I would read this targeting mechanic.
X: empty square
P: PAL
W: Wall of Force
O: Point of Origin
C: Caster
CXXOWP
The burst goes off in O, fails to permeate the W, and therefore P is not a Target of the power. Therefore DC goes off dealing damage but wait does DC need line off effect?
To answer your question Hyp, the caster of the fireball intended to include the P in the fireball but in the end P wasn't a target of the fireball, so I'd say it was an attack that didn't include the P.
Damn, shame on me...your example assumes P=O.
CXXXWP P=O
Now when I look at PHB P273 Line of Effect, "You need LoE to any target you wish to attack and to any space in which you wish to create an effect."
So you can't create the O there because you don't have LoE, but you don't know that you don't have LoE (Because nowhere is stated in the rules that you automatically know whether you have LoE or not.).
I have perfectly no idea what the RAW says on this problem.
I'd probably (house-) rule: you fail to cast your spell, but keep it, action wasted. => therefore no attack that didn't include P, no DC.
Other variant: You try to cast but fail (lack of LoE), daily used, no effect (only intent), no attack made, no attack made that did not include P, no DC.