In a kind of strange synergistic occurance, both Evan DiBiase over at 32584.com and I have been sleuthing (Evan’s “Nancy Drew” reference!) for new Yahoo!Sync references in OS X applications, specifically Address Book. My strings search (strings */Contents/MacOS/* | grep -i yahoo) found a good number of new Yahoo references, which you can examine after the jump. What’s more, Evan also found what completely escaped my notice: a Yahoo end-user licensing agreement in Address Book. (It’s in Contents/Resources/English.lproj/YEULA.html) Pop over to 32584.com to keep up to date on his discoveries.
Yahoo EULA in Address Book
By clicking “Agree” below, you will enable your device to access your Yahoo! account for the purpose of synchronizing, as applicable and if available, your data in your Yahoo! Address Book and other Yahoo! services with the relevant data on your device. You acknowledge and agree that the synchronization process may read from, change, delete, or overwrite data in your Yahoo! account and/or your device. Yahoo! strongly recommends that you make a backup copy of your data before your first synchronization attempt. Yahoo! assumes no responsibility, makes no representations or warranties, and shall not be liable for any damages whatsoever for lost or duplicated data. If you change your Yahoo! log in credentials, you may be required to give permission again.
Yahoo strings in Address Book’s executable
com.apple.AddressBook.spotlight.yahoo yahoo Yahoo YahooSyncSupport ABYahooSyncPrefsController toggleSyncWithYahoo: yahooEULASheetDidEnd:returnCode:contextInfo: showYahooEndUserLicenseAgreement: yahooSyncSetupSheetDidEnd:returnCode:contextInfo: editYahooSyncPrefs: shouldShowYahooSyncSettings setupYahooSyncSettings initializeYahooSyncFromDefaults yahooSyncPrefsController syncWithYahoo editYahooSyncPrefs yahooPrefsSeparator _yahooSyncPrefsController _isEnablingYahooSync _yahooAvailabilityCheckCompleted _yahooAvailable @"ABYahooSyncPrefsController" ABYahooSyncSetup


This might be worth playing with:
grep -rail yahoo /Applications /Library /System | egrep -vi '(da|Dutch|fi|French|German|Italian|Japanese|ko|no|pt|Spanish|sv|zh_CN|zh_TW)\.lproj'
That will scan *everything* for references to Yahoo, while filtering out the non-English localization bundles which will probably mostly be duplicates of the English language information.
Off the bat, I see mentions of "yahoo" in Address Book, iChat's help section, iPhoto (mention of http://search.yahoo.com/mrss), iWeb, Keynote, Pages, Safari, the Apache documentation, etc.
Admittedly, some of this was probably already there prior to 10.4.10, but still, this should be a good general way to get a list of files to inspect.
I see YahooSyncSupport
This makes me think defaults write com.apple.AddressBook YahooSyncSupport -bool true
However, that doesn't do anything. Oh well. Maybe it's time I installed F-Script.