\documentclass[a4paper]{article}

\usepackage{pgfplots}
\pgfplotsset{compat=1.3}
\begin{document}

\begin{tikzpicture}
	\begin{axis}[ybar,symbolic x coords={a,b,c}]
	\addplot coordinates {(a,10) (b,20) (c,30)}
		node[pos=0.5] {\pgfplotspointplotattime \pgfkeysvalueof{/data point/x}}
		;
	\end{axis}
\end{tikzpicture}
\end{document}

