For those interested, my good friend Justin Bakse has just released a handy application named Ligature Counter. In short, you can paste text into the application and get results for both letter frequency and letter combo frequency. Very nice for analyzing text, thinking about frequent letterforms or working out some new ligatures. The application is free and web based.
Take it for a spin here:
http://www.volcanokit.com/volcanokit2/ligCounter/
Many thanks to Justin for making this for me (and then making it free for everyone) and Hrant for planting the initial idea in my head for such a thing.
Best,
Eric O.