Are sushi rice and arborio rice valid substitutes of each other?
I've done risotto with other short grain rices (which includes sushi rice), with good results.  I've even used medium grain rice in a pinch, and you'll have to stir more, but it'll come out.

I'd assume the same would be true in reverse -- that most short grain rices (incl. arborio) would make a decent sushi ... unless it's short grained brown rice, that'd likely less you up.

Basically, there's three main varieties of rice:  short, medium and long  (not including 'wild rice' or how it's treated (white vs. brown vs. parboiled).  So long as you stay within one group, you're usually fine, so long as you don't change the treatment or technique used to cook it.