here's an approach that uses labels for the symbols and \pageref to keep them in input order. the \pagerefs need to be expanded before writing to the .idx file; ordinarily index material is written out verbatim. this even permits multiple page references to the same symbol; in such a case, only the first appearance should get a label.
the routine needs some more work, since at the moment it trims off leading zeros from \pageref numbers, so if there are any entries on pages 1-9 they will be out of order in an index for a book of up to 99 pages, etc. (it would probably be best to normalize on 3-digit page numbers.) i'll work on that when time permits, but if someone else gets there first, go for it!
i'm also not sure what would happen with a symbol first introduced in a preface, or on some other page numbered with roman numerals. another open problem.
\documentclass{book}
\usepackage{makeidx}
\makeindex
\newcommand{\xindex}[2]{\expandafter\index\expandafter{\pageref{#1}#2}}
\begin{document}
\chapter{One}
some text $Z$\label{not:Z}\xindex{not:Z}{@$Z$} some more text.
\newpage
some text $C$\label{not:C}\xindex{not:C}{@$C$} some more text.
\newpage
some text $P$\label{not:P}\xindex{not:P}{@$P$} some more text.
and a second reference to a symbol that first appeared on a previous page
$C$\xindex{not:C}{@$C$} some more text.
\printindex
\end{document}
It is always best to compose a fully compilable MWE that illustrates the problem including the
– Peter Grill Jan 23 '12 at 17:44\documentclassand the appropriate packages so that those trying to help don't have to recreate it. I realize it is not complicated but any time you can save those trying to help would be helpful.