Use Grapher to embed equations in Pages and Keynote: "There is no equation editor available in Keynote or Pages, but it is possible to creat nice equations in the Grapher application (in /Applications » Utilities) included in OS X, and then copy/paste the equation(s) into Pages or Keynote.
However, I don't know how to select a font color to match Keynote's master. Perhaps there's a better/free way to create nice equations to use in Keynote and Pages?
(Via Mac OS X Hints.)