How can I change how OS X's 'say' command pronounces a word?

Well you can also input the exactly phonemes you want to be said. The syntaxe is not so complicated.

Ex:

[[inpt PHON]] hAAIH.

Check this link to learn more: http://developer.apple.com/library/mac/#documentation/UserExperience/Conceptual/SpeechSynthesisProgrammingGuide/Phonemes/Phonemes.html#//apple_ref/doc/uid/TP40004365-CH9-SW1

• Create a list of word pairs [spolsky spowlsky; joel jole; ...].
• Create a script in your favorite scripting language that substitutes words in its parameter list according to your word-pairs list, and passes the modified parameter list to "/usr/bin/say".
• Make the script executable, call it "say" and put it in a folder that appears earlier in your $PATH than does "/usr/bin".
• Add spelling variants to your word-pairs list as you find more words you'd like pronounced differently.

Your new say will work like the old say, but with your preferred pronunciation.