LaTeX is the perfect application for generating well-typeset equations, but it isn't easy to get the output into Keynote. Here's a script that will help you do this (you may want to install Fink to get LaTeX and the epstopdf command).
#!/usr/bin/perl -w
use strict;
my $equation=' ';
while (<STDIN>) {$equation .= $_}
open (OUT,'> /tmp/eq.tex');
# The next line was broken into two for easier web display
# Enter the two rows as one, and don't enter this comment...
print OUT "\\documentclass{article} \\usepackage{pslatex} \\begin{document}
\\pagestyle{empty} \$$equation\$ \\end{document}";
close OUT;
system "cd /tmp; latex eq";
system "dvips -E /tmp/eq.dvi -o $ARGV[0]";
unlink '/tmp/eq.tex','/tmp/eq.dvi','/tmp/eq.aux','/tmp/eq.log';Save this to somewhere like your home directory, calling it keynoteeq or similar, and run chmod u+x keynoteeq in the terminal to make it executable. Then from that directory you can run ./keynoteeq output.eps. Type in the LaTeX code you want (leaving out the surrounding $s or \begin or \end{equation}s - use Google for help on LaTeX), hit Enter and then Contrl-D. The program will save an encapsulated postscript file to output.eps, which you can convert to pdf using epstopdf output.eps. Then just drag and drop the output.pdf file you've created into Keynote.
This could be made even easier if people want - but I'll let someone do that in the comments!
[robg adds: I haven't tested this script...]

