I'm no expert, but here are my 2cents:

I'm not sure, but I recall that for a six layer board, the center two layers
are on either side of the 'core' material, which is the thickest part of the
sandwich (~=30mil?).  This is the most popular (and most appropriate) place
for the power and ground plane, but causes the least inter-plane capacitance
due to this thickness.  There may not be much. (calculations anyone?)

Moving to the next layer pair out is worse - more distance apart, with
signal layers in-between.  Moving the planes to one side or the other
produces problems (bananas).

The next step is an 8-layer board with two power-ground plane pairs - that
way the pairs only have the thin laminate layer between greatly increasing
the capacitance between the planes.  At one time I think there was an
enhanced dielectric laminate material available to increase this effect
further.

Regardless, I think that a feedthrough has much greater inductance that the
lead inductance of an 0805 (or smaller) capacitor.  When you get into
extremely high frequencies, you just have to pay more attention to part
selection - chose a part value that has self-inductance (self-resonance)
appropriate for the intended use. (i.e. 27-100pF for 900MHz)

Always select the path of least inductance.  If the power and ground pins
are adjacent, use the shortest traces to an APPROPRIATE cap, then sink
feedthroughs to the power/ground plane (on the far side of the cap).  I'm
sure that the combined trace/lead inductance will be less than that of a
feedthrough.  Then, ABSOLUTELY let the parasitic effect of inter-plane
capacitance provide additional (not replacement) decoupling.

If the power and ground pins are far apart, then a feedthrough might have
less inductance (as well as reducing routing problems) than running traces
to a cap.

There are no absolutes.  Use the most appropriate method for each need!



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