Multiline equation with LHS alone on first line?

Ok, converting my comment on @egreg's solution into an answer:

\begin{align*}
  \hspace{2em}&\hspace{-2em}Any short or long LHS\\
  &= ax^3 + bx^2 + cx + d \\
  &= f(x)
\end{align*}

This puts the first LHS on a separate line, and the equation is centered correctly, independent of the size of LHS.

Additional note: I've defined a shorthand command for \hspace{2em}&\hspace{-2em} myself. I wasn't sure that putting the & into a command would work, but it does.

this should do it.

\documentclass{article}
\usepackage{amsmath}
\multlinegap=30pt
\begin{document}
\begin{multline*}
 \text{Left-hand-side of my equation}\\
  \begin{aligned}
    &= \text{right-hand-side number 1}\\
    &= \text{right-hand-side number 2}\\
    &= \text{etc.}
  \end{aligned}
\end{multline*}
\end{document}

Edit: \multlinegap specifies the indent from the left and right margins. the default is 10pt, but often a wider gap is desirable.

Simpler than Werner's but using the same idea for centering with respect to the supposed long left hand side:

\begin{align*}
  \makebox[2em][l]{\text{Here is your LHS that may be very long}} &
  \hspace{-2em}\hphantom{\text{Here is your LHS that may be very long}}\\
  &= ax^3 + bx^2 + cx + d \\
  &= f(x)
\end{align*}

If instead it's one of the right hand sides to be overlong, one can use

\begin{align*}
  \makebox[2em][l]{\text{Not so long LHS}} & \\
  &= ax^3 + bx^2 + cx + d + ay^3 + by^2 + cy + d \\
  &= f(x,y)
\end{align*}