Split environment alters spacing around binary operators

The usual way of using the alignment operator is either & = (on the left side), or via ={} & (to correct the spacing around the binary relation. Here's how you would use both:

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

\begin{equation}
  f(x) = m x^2 + b
\end{equation}

\begin{equation}
  \begin{split}
    f(x) = & m x^2 \\
    & + b
  \end{split}
\end{equation}

\begin{equation}
  \begin{split}
    f(x) & = m x^2 \\
    &\phantom{{}={}} + b
  \end{split}
\end{equation}

\begin{equation}
  \begin{split}
    f(x) ={} & m x^2 \\
    & + b
  \end{split}
\end{equation}

\end{document}

Note how the correction for the \phantom relation = ensures proper spacing.

you're not following the recommended/required syntax. from the manual (texdoc amsldoc):

In the structures that do alignment (split, align and variants), relation symbols have an & before them but not after -- unlike eqnarray. Putting the & after the relation symbol will interfere with the normal spacing; it has to go before.