Hm, I was going to give the same link as Mark, but personally, I would use the graphic, and the technique in Mark's link. this is called "image replacement", so you can Google that for other approaches, though the method in that ink is the best.
Another half-way option is to include the real text but move it way off screen. That covers the search engines, but it does mean that people will see no text at all if images are disabled.
I agree that loading a whole font for a few words is overkill.