How to center text without adding space AND not altering alignment of surrounding text?

How about this:

\documentclass{article}

\begin{document}

This is a really long sentence as an example.  The second have of this\\
\centerline{sentence should be centered.}
\end{document}

Please do not use \centerline if possible, it's not suitable for long text. Just patch LaTeX's center environment like this:

\documentclass{article}
\newenvironment{tightcenter}{%
  \setlength\topsep{0pt}
  \setlength\parskip{0pt}
  \begin{center}
}{%
  \end{center}
}

\begin{document}
text text text text text text text text text text text text text text text text text text text text text text text text
\begin{tightcenter}
foo
\end{tightcenter}
text text text text text text text text text text text text text text text text text text text text text text text text

\end{document}

You could use the TeX primitives \rightskip and \leftskip; a little example:

\documentclass{article} 

\begin{document}

\begingroup
\leftskip=0cm plus 0.5fil \rightskip=0cm plus -0.5fil
\parfillskip=0cm plus 1fil
This is a really long sentence as an example.  The last line of this
paragraph will be centered.\par
\endgroup
Another sentence that starts a new paragraph

\end{document}

The explanation of the code (as given in TeX by Topic):

For all lines of a paragraph but the last one the stretch components add up to zero so the \leftskip and \rightskip inserted are zero. On the last line the \parfillskip adds plus 1fil of stretch; therefore there is a total of plus 0.5fil of stretch at both the left and right end of the line.