Q:

Center table by end of cell

I want to be able to center my table, but align the center by the end of the first cell. Currently just using the \begin{center} command i am able to center the table but i would rather the table be centered based off the end of the first cell and not by the average center of the table.
\documentclass{article}  
\begin{document}  

\begin{center}
\begin{tabular}{r | l}
I am some really long text & 10pt \\ 
I am also really long & 12pt \\ 
\end{tabular}
\end{center}
\end{document}  

Gets a table that is centered, but i would rather it went
"I am some really long text {center here} 10pt.
Edit: The idea i'm kind of looking for can be seen in the way that you can align equations at any point.

A:

Just a variant of Poul's answer. Instead array package can use tabularx package and define new column types R and L. There is no need to measure width of columns:
\documentclass{article}
\usepackage{tabularx}
\newcolumntype{L}{>{\raggedright\arraybackslash}X}
\newcolumntype{R}{>{\raggedleft\arraybackslash}X}

\begin{document}
\begin{center}
  \begin{tabularx}{\textwidth}{|R|L|}
    I am some really long text & 10pt \\
    I am also really long      & 12pt 
  \end{tabularx}
\end{center}  
\hrulefill.\hrulefill % just for visual check
\end{document}  

A:

I don't know why Chris (cmhughes) deleted his answer. But, for this simple job, no need of loading a heavy package like tabularx. Without it, this is how it is done.
\documentclass{article}
\usepackage{showframe}
\usepackage{array}
\newcolumntype{L}{>{\raggedright\arraybackslash}p{\dimexpr0.5\textwidth
                                                   -2\tabcolsep-\arrayrulewidth\relax}}
\newcolumntype{R}{>{\raggedleft\arraybackslash}p{\dimexpr0.5\textwidth
                                                   -2\tabcolsep-\arrayrulewidth\relax}}
\begin{document}

\begin{center}
    \begin{tabular}{R|L}
    aaaa    I am some really long text & I am some really long text aaaa\\
        I am also really long      & I am also really long     \\
    \end{tabular}
\end{center}
\hrulefill.\hrulefill % just for visual check
\end{document}

A:

Obviously, extra columns could be inserted on either (sub)tabular to extend the table leftward or rightward, respectively, even as the dividing line between the halves remains centered.
\documentclass{article}  
\begin{document}  

\begin{center}
\makebox[0pt][r]{%
\begin{tabular}{r |}
I am some really long text\\ 
I am also really long\\ 
\end{tabular}%
}%
\makebox[0pt][l]{%
\begin{tabular}{| l}
 10pt \\ 
 12pt \\ 
\end{tabular}%
}
\end{center}
\end{document} 

