Be careful with Yahoo Messenger 3.0 beta 1 on Tiger (I am using 10.4.7), it has erased my address book twice. This appears to happen sometimes when you open Yahoo Messenger and your AddressBook is open and (I think) using spotlight to update it's indexes. After your address book vanishes, you will need to go to your ~/Library/Appication Support/Address Book folder and replace the AddressBook.data with AddressBook.data.previous (but keep a copy of both). This can be tricky for other reasons because there will be processes hung and spotlight indexing might be running out of control. I have had some cumbersome experiences getting my address book back.

This is why it is called Beta, people. If you risk running Beta, you risk loosing data and functionality will not be complete. I applaud Yahoo's efforts - but this bug is a real killer.