I’ve had a couple of babel-related bug reports for siunitx. The first was with the Spanish language option, where the
\percent unit misbehaves as
\% is redefined by babel. My fix for this has then caused a problem with the Italian language option, as it makes ” an active character. So the latest release (v1.2d) fixes this too: apologies for the problems.
All of this makes more more eager than ever that a general move to UTF-8 input happens. This should cut down on the need for babel. At the same time, an updated LaTeX kernel (LaTeX3) which includes more functionality would also help package authors know what to expect.