My Molly has the same.... it's just skin pigmentation as a result of cronic infection. Peanut might have had a yeast infection or some kinda of pyroderma (sp? - that's a skin infection) in the past, it would be gone now but the pigmentation stays.
Nothing to worry about... just watch for redness and itchies as signs of new infection. Peanut is probably prone to them. Let the vet know though - so they can double check for signs of infection
Magnolia (aka Nolie) - 6 yrs old, adopted May 3, 2009 - mixed breed monster mutt
R.I.P. Chloe - my first Golden girl - we said goodbye but you will always be here (1994-2007 adopted April 2000)
R.I.P. Molly - my Golden girl, who's causing havoc over the bridge now - (1999-2009 adopted May 2007)
and love for my "lost ones" - Misery, Clover & the girls (chickens)