Ausoni-Bokstedt duality for topological Hochschild homology

We consider the Gorenstein condition for topological Hochschild homology, and show that it holds remarkably often. More precisely, if R is a commutative ring spectrum and and R----->k is a ring map to a field of characteristic p then, provided k is small as an R-module, THH(R;k) is Gorenstein in the sense of Dwyer-Greenlees-Iyengar. In particular, this holds if R is a (conventional) regular local ring with residue field k of characteristic p. Using only Bokstedt's calculation of THH(k), this gives a non-calculational proof of dualities observed in calculations by Bokstedt, McClure-Staffeldt, Ausoni-Rognes, Ausoni, Lindenstrauss-Madsen, Angeltweit-Rognes and others. A lemma of Dundas shows that THH(R;k) is remarkably computable.