I'm trying to place 4 subfigures of different sizes aligned. Here the box-k.pdf files contain boxes with marks at the midsides. The result has box-1 and box-3 aligned at the left margin, not centered (but the captions are centered).
texlive-2012, as distributed for Fedora.
\documentclass{memoir}
\usepackage{pgf}
\usepackage{subfig}
\begin{document}
\begin{figure}[htbp]
\setbox1=\hbox{\pgfimage{box-1}}
\setbox2=\hbox{\pgfimage{box-2}}
\setbox3=\hbox{\pgfimage{box-3}}
\setbox4=\hbox{\pgfimage{box-4}}
\centering
\subfloat[Box 1]{
\begin{minipage}[c]{1.0\wd3}
\begin{center}
\copy1
\end{center}
\end{minipage}
}
\hspace{4em}
\subfloat[Box 2]{
\begin{minipage}[c]{1.0\wd4}
\begin{center}
\copy2
\end{center}
\end{minipage}
}
\\[2ex]
\subfloat[Box 3]{
\begin{minipage}[c][1.0\ht4]{1.0\wd3}
\begin{center}
\copy3
\end{center}
\end{minipage}
}
\hspace{4em}
\subfloat[Box 4]{
\begin{minipage}[c]{1.0\wd4}
\begin{center}
\copy4
\end{center}
\end{minipage}
}
\caption{Some boxen}
\end{figure}
\end{document}

subcaption, notfloatrow– cmhughes Nov 20 '12 at 15:57