Center-justify with hrule above and below

Pilfering from @egreg's answer... You could modify the inter-word spacing or just add a couple of \hfills inbetween:

\documentclass{article}
\usepackage{booktabs}% http://ctan.org/pkg/booktabs
\begin{document}

\mbox{\huge\bfseries
  \begin{tabular}{@{}c@{}}
  \toprule[.25mm]
  This is my title, it is somewhat\\
  long\hfill and\hfill split\hfill over\hfill two\hfill lines!\\
  \bottomrule[.25mm]
  \end{tabular}%
}

\end{document}

On its own, \hfill will do nothing inside a tabular if there isn't enough room to stretch. However, since the first line is longer than the second, there is room to stretch.

---

\makebox[<len>][<justification>]{<stuff>} can also be used with <justification> given by [s] (for stretch) if you know the <len> that <stuff> should be spread over:

\documentclass{article}
\usepackage{booktabs}% http://ctan.org/pkg/booktabs
\begin{document}

\newlength{\mylen}
\noindent{\huge\bfseries
  \settowidth{\mylen}{This is my title, it is somewhat}%
  \begin{tabular}{@{}c@{}}
  \toprule[.25mm]
  This is my title, it is somewhat\\
  \makebox[\mylen][s]{long and split over two lines!}\\
  \bottomrule[.25mm]
  \end{tabular}%
}

\end{document}

Use a tabular environment:

\documentclass{article}
\usepackage{booktabs}
\begin{document}
\mbox{\huge\bfseries
  \begin{tabular}{@{}c@{}}
  \toprule[.25mm]
  This is my title, it is somewhat\\
  long and split over two lines!\\
  \bottomrule[.25mm]
  \end{tabular}%
}
\end{document}