The 0 appears centered, as it should. The infinity sign, however, is over the leftmost integral operator -- not centered. How can I get it to be centered?
A quick fix: add manually some space, writing something like \iiint_0^{\hspace{6pt}\infty}. Anyway, are you sure you need this notation? For me, this is the first time I see something above the triple integral sign.
Maybe you've found a bug in the default Computer Modern math font package. Because if you switch to a different math font package, like txfonts or fourier, the infinity sign is centered along the top: