Fascinating, Georgiansilver. But it has one S too many - it should be "SATOR AREPO OPERA ROTAS", otherwise it doesn't work out. Indeed, in the picture on this page you can see the spelling clearly. The thing about this phrase is that it is a "square palindrome"; when written one word below the other, it reads the same left to right, right to left, top to bottom and bottom to top.

S A T O R
A R E P O
T E N E T
O P E R A
R O T A S

Apparently (I just read the accompanying text on that page) the early Christians used it as a secret communication between them, as it contains the letters for "PATER NOSTER" (our Father) twice, leaving over only the letters A and O twiice (as in "the Alpha and the Omega", referring to God).
And though it appears in Naturalis Historia by Pliny (who died in 79AD), it looks like the inscripition is somewhat earlier.