Reference source code line in minted package

Use mathescape, then put your \label in math mode in a comment:

\begin{minted}[linenos=true, mathescape]{c++}
    i = i + 1 ;  
    j = j + 1 ; // The important line $\label{myline}$
    k = k + 1 ;
\end{minted}
The important line is line \ref{myline}.

@Ant's answer is great, but requires that your \label go inside a comment that will appear in the rendered code. You can get around this by using escapeinside instead of mathescape:

\begin{minted}[linenos=true, escapeinside=!!]{c++}
    i = i + 1 ;  
    j = j + 1 ; !\label{myline}!
    k = k + 1 ;
\end{minted}
The important line is line \ref{myline}.

If you are happy to change from minted to listings then it is possible as this example shows:

\documentclass{article}
\usepackage{listings}
\lstset{basicstyle=\ttfamily,
  escapeinside={(*}{*)},
  numbers=left
}
\begin{document}
\begin{lstlisting}
  def foo
    while do bar(*\label{line}*)
  end
\end{lstlisting}
As we can see in line \ref{line}
\end{document}

Unfortunately, this means a lot more work to get nice colourful syntax highlighting. escapeinside defines a way to escape the verbatim environment and have what's inside the (* and *) actually read by TeX...