Typesetting plus-minus uncertainties with different upper and lower bounds

Do you mean something like

\documentclass{article}
\usepackage{amsmath}
\begin{document}
 $1.23\substack{+0.4 \\ -0.5}$\,pb
\end{document}

(I've been asked about this for siunitx, but have never really worked out what it actually means or a good interface for an 'automated' approach.)