Inspired by Werner's, here's a definition that works with hyperref
\documentclass{memoir}
\usepackage[demo]{graphicx}
\newsubfloat{figure}% Allow subfigures using \subbottom/\subtop
\usepackage{refcount}
\newcommand\sref[1]{\edef\next{\getrefnumber{#1}}%
\begingroup\edef\x{\endgroup
\noexpand\hyperref[#1]{\expandafter\stripparens\next()\nnil}}\x}
\def\stripparens#1(#2)#3\nnil{\if\relax\detokenize{#2}\relax??\else#1#2\fi}
\usepackage{hyperref}
\begin{document}
\begin{figure}[ht]
\centering
\subbottom[This is a subfigure\label{fig:label:a}]{\includegraphics{figure1}} \qquad
\subbottom[This is a subfigure\label{fig:label:b}]{\includegraphics{figure2}}
\caption{This is a caption} \label{fig:label}
\end{figure}
See Figure~\ref{fig:label}. It has subfigures~\sref{fig:label:a} and~\sref{fig:label:b}.
\end{document}
If hyperref is not loaded, then change the definition of \sref into
\newcommand\sref[1]{\edef\next{\getrefnumber{#1}}%
\expandafter\stripparens\next()\nnil}
It's a kludge and requires \sref instead of \ref, but it seems to work. When memoir will be adapted so that the aspect of references to subfloats is customizable, it will be necessary only to say
\let\sref\ref