[XeTeX] Converting math symbols via xetex

[Neil]

Sorry if this is a common question, I've searched high and low for the answer.

I am writing a scientific paper with math terminology. It would be
nice to directly enter some math symbols directly into the
text-editor. For example, the characters
φ (lower case phi aka $\phi$)
or
¬ (negation aka $\neg$)

It makes it clearer to read the source, I find.

My text file is UTF-8. If I run xelatex on my Mac 10.6, the resulting
pdf does show the φ character, but not the ¬ character. I cannot
understand why. My feeling is that it has something to do with
font-mapping.  I've included my document header for reference.

thanks for any help.

XeTeX 3.1415926-2.2-0.9995.2 (TeX Live 2009)
kpathsea version 5.0.0
Copyright 2009 SIL International and Jonathan Kew.
There is NO warranty.  Redistribution of this software is
covered by the terms of both the XeTeX copyright and
the Lesser GNU General Public License.
For more information about these matters, see the file
named COPYING and the XeTeX source.
Primary author of XeTeX: Jonathan Kew.
Compiled with ICU version 3.8.1 [with modifications for XeTeX]
Compiled with zlib version 1.2.3; using 1.2.3
Compiled with FreeType2 version 2.3.9; using 2.3.9
Using Mac OS X Carbon, Cocoa & QuickTime frameworks


%!TEX TS-program = xelatex
%!TEX encoding = UTF-8 Unicode
\documentclass{llncs}
\usepackage{todonotes}
\usepackage[no-math]{fontspec}
\usepackage{xunicode}
\usepackage{xltxtra}
\setmainfont[Mapping=tex-text]{Times}
%\usepackage{amsthm}
\usepackage{color}
\usepackage{booktabs}

\usepackage{graphicx}
\newcommand{\slbl}[1]{\textbf{\textsf{#1}}}
\usepackage{amsmath}%,amssymb}
\usepackage{algpseudocode}
\usepackage{algorithm}%,algorithmic-fix}
\usepackage[pdfstartview=FitH,xetex,bookmarksopenlevel=3,bookmarks=true]{hyperref}
%pdfpagescrop={92 112 523 778},

\begin{document}