categories: Only two SMC structures on Cat?

[Jason C Reed <jcreed@andrew.cmu.edu>]

Power, A.J. and Robinson, E.P. Premonoidal categories and notions of
computation (ftp://ftp.dcs.qmw.ac.uk/pub/lfp/edmundr/premoncat.ps.gz) in
section 2 asserts that there is excatly one symmetric monoidal closed
structure on Cat besides the cartesian one. Does anyone know [the location
of] a proof?

---Jason