> Or is there something really simple that I've missed?

According to Plansescape a priest is hampered while on planes not of his
deity. Traveling around the Great Ring (the order of the planes and the
ring is roughly the same from 1st through 2nd, though the names differ)
find the shortest path (without going through the Outlands/Concordant
Opposition) between the priest and the god. The number of steps is the
number of effective experience levels the priest loses in regards to spell
casting.

However, the Prime is effectively 2 steps from any outer plane (Home,
Astral, Prime) but there is NO drop in power. The only thing that planar
scholars can explain is "the powers want it that way, berk", or something
to that effect.

In essence, if the gods of Aebyrnis didn't want their followers hampered
by the Shadow World, then they wouldn't be.

l8r
Tim Nutting