check out these links:

http://www.rebelwheels.com.au/wheel_care.html

http://www.iauto.co.uk/pages/alloy_wheel.htm

http://www.finishing.com/84/90.html

i'm not advertising or making any claims for any specific product...just did
a yahoo keyword search for "aluminium wheel pitting auto".

Many cast aluminum wheels like those on the Sorento are coated with a clear
acrylic coating in order to prevent pitting.  There is a possibility that
what you could be experiencing is from either the clearcoat on the wheels
not being applied, not applied properly, or something happened to make it
fail.  I'm sure that there are any number of products or processes that will
either remove the pitting or remove the brake dust build-up in the pitting,
but to prevent it from continuing I think you will most likely have to have
the wheels shot with another coat of sealant, otherwise it's just going to
start all over again.  You don't mention where you live or what climate, but
I've heard that a winter's worth of road salt can cause this type of
problem, or perhaps someone buffed the wheels in order to make them shiny
but regrettably removed the clear coating on them.  In order to help prevent
a reoccurance of this in the future, I suggest a set of dust shields for the
inside of your rims to help keep the brake dust off of your wheels.

Cheers - Jonathan