The Dictionary app, which debuted in 10.4, offers the ability to display pronunciations in (among other choices) US English (IPA) (International Phonetic Alphabet) in its preferences.
Unfortunately, the pronunciations given have some glaring errors. For example, the symbols for primary and secondary stress are swapped, and the symbol for the vowel sound of "cat" and "plan" is incorrect.
Thankfully, Apple had the forethought to keep the dictionary in an unencrypted XML format in /Library -> Dictionaries -> New Oxford American Dictionary.dict -> Contents -> dict_body. Thus, the following command will fix the dictionary in place:
$ perl -pi -e 's/\303\270/\303\246/g; tr/\313\214\313\210/\313\210\313\214/'\
/Library/Dictionaries/New\ Oxford\ American\ Dictionary.dict/\
Contents/dict_body
Use -pi.bak (instead of -pi, after perl) if you want to make a backup of the dict_body file, which is 75 megabytes.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050823032821731