Sascha Brawer just posted to the fonttools list:
we [Google] just released fontdiff, a small utility for testing fonts. When you modify a TrueType or OpenType font, fontdiff generates a PDF showing the typeset text both before and after the change. You can use this PDF to easily review the changes and spot any errors caused by a font switch. For every line in the text sample, the tool renders two raster images in high resolution. One image is typeset using the original font, and the second uses the new font. If there is any difference between these two high-resolution images (even if it’s just whitespace caused by kerning changes), the output PDF highlights the difference in color.
https://github.com/googlei18n/fontdiff