A few days ago I upgraded from bitcoin 0.4.x to bitcoin-qt 0.5.1 on Mac... I used the DMG linked from bitcoin.org... the software identifies in its about box as "0.5.1-beta".
Now, when I look in 'Address Book', numerous addresses which were locally created and have received bitcoins don't appear.
However, my balance is correct, and from the transaction log, I can double-click on those previous receive transactions to get details. In those details other addresses that are properly identified as mine and labeled appear.
Updated! Nevermind, I see receiving addresses have been moved out of the address book to the receive-coins button.