siunitx S column to produce aligned numbers in italics and parentheses

Second approach

This approach avoids abusing the nature of digits!

\documentclass{article}
\usepackage{siunitx}
\DeclareRobustCommand\tnote[1]{\textsuperscript{\emph{#1}}}
\begin{document}
\begin{tabular}{
  S[math-rm=\mathit]
  S[math-rm=\mathit,table-format=1.3,table-align-text-post = false,
    input-open-uncertainty=,input-close-uncertainty=]
  }
1&(1.23)\tnote{a}\\
2&(2.012)\tnote{b}
\end{tabular}
\end{document}

First approach

\documentclass{article}
\usepackage{siunitx}
\begin{document}
\begin{tabular}{
  S[math-rm=\mathit]
  S[math-rm=\mathit,table-format=2.3,input-symbols=()]
  }
1&(1.23)\\
2&(2.012)
\end{tabular}
\end{document}

which is a slight refinement of egreg's solution. I can't at the moment thing of a way to auto-include the parentheses.

(The bracket-numbers option is about cases where the output may be ambiguous, and therefore brackets are used to keep it mathematically correct.)

Try this:

\begin{tabular}{S[math-rm=\mathit]
  >{{(}}S[math-rm=\mathit,
          table-format=1.3]<{{)}}}
1&1.232\\
2&2.012
\end{tabular}