siunitx: aligning numbers by decimal points in tables doesn't work for bolded or italicized numbers

You have to use the detect-all package option and to add \bfseries to the column declaration instead of using \textbf for every cell. (Sorry for not answering before - I wrongly assumed you wanted a single cell in bold.)

\documentclass{article}

\usepackage{booktabs}
\usepackage[detect-all]{siunitx}

\begin{document}

\begin{table}
  \begin{tabular}{@{}lS[table-format=3.2]>{\bfseries}S[table-format=3.2]@{}}
    \toprule
    \textbf{Foo} & \textbf{Normal} & \textbf{Bold} \\
    \midrule
    foo1 & 111 & 111 \\
    foo2 & 222.2 & 222.2 \\
    foo3 & 3.33 & 3.33 \\
    foo4 & 4 & 4 \\
    foo5 & 5.5 & 5.5 \\
    \bottomrule
  \end{tabular}
\end{table}

\end{document}

this came up recently in another forum and Joseph came up with a solution. I include his aswer. the key is to use the etoolbox package and \robustify\bfseries

Okay, the way that siunitx works with formatting is a little complex as it has to pick up which macros to expand and which not to. As e-TeX is required, I've taken the attitude that things 'not to expand' can be e-TeX \protected. For the case in hand, you therefore need to make `bfseries` robust. Something like

\documentclass{article}
\usepackage{colortbl,etoolbox,siunitx,xcolor}
\robustify\bfseries \sisetup{
detect-weight = true , locale = DE , }
\begin{document}

\begin{table} \sisetup{
group-separator = ., table-format = 5.0,
table-number-alignment = right , }
\begin{tabular}{S[table-format = 8.0,
table-number-alignment = right]SlSS} \rowcolor[gray]{0.9}
\bfseries 1234567 & \bfseries 12345 &
\bfseries Test & \bfseries 12345 &
\bfseries 12345 \\ 1234567 & 12345 &
Test & 12345 & 12345 \\
\end{tabular}
\end{table}

\end{document}

should do the job. I've got a bug fix of siunitx to do, so I'll add a note on this to the documentation. Let me know if the above works for you.