I had some trouble reading nested IF functions, so I wrote a piece of code which translates them into flowcharts: http://excel2flow.asp2.cz/ (weekend project)
It briefly bugged out (dumping everything as a string into a single result block) on entering a ":" or ";" after the word "Error" in a string as part of the long formula I tested it with, but since then I've changed a few minor things and it's stopped happening :S
Anyway, I wrote the parser by hand so I cannot guarantee it will work on every valid Excel formula. But I'm working on improved version which will use formal grammar.