[nq:1]I think "icing on the cake" is more common in AmE, at least in my experience. I believe that icing and frosting are somewhat different categories (in AmE), but I'll defer to the experts.[/nq]
I'm (BrE) familiar with this, usually as "THE icing on the cake". Without the definite article, I'd expect something more along the lines of, "...but that's just... ".
Cheers - Ian