Proceed with this how-to with caution and a good backup…
In order to synchronize your Apple AddressBook using Dropbox you need to have Dropbox installed on all the computers you wish to sync, and you need to have some comfort running commands in Terminal. This synchronization is relatively easy to accomplish, but missing a step or mistyping a command could leave you without ready access to your address book. Make backups and follow the steps carefully and you should be okay.
Get a copy of Dropbox if you haven’t already. It’s an amazingly useful tool that gives you 2 GB of storage on the “cloud” for free. With clients for Linux, Windows, and Mac operating systems, I use it every day. The installation and setup is well explained on the Dropbox site so I won’t cover that here.
With Dropbox installed on each machine you wish to sync you need to prepare and backup your AddressBook. To prepare my Address Book I combined all my addresses into a single address book. This combined address book is the one I pushed to Dropbox. But before that I made backups of all my address books. Under File | Export you’ll find the Address Book Archive… option.
Once your backups are completed, you are ready to move the combined Address Book to Dropbox. To do this you will use the Terminal application located in the Applications | Utilities folder. Make sure that you have quit Address Book before doing these commands. Also, these commands assume your Dropbox is located in your home folder. If you placed it elsewhere, you’ll have to adjust the Dropbox path accordingly.
$ mv ~/Library/Application\ Support/AddressBook ~/Dropbox/
$ ln -s ~/Dropbox/AddressBook ~/Library/Application\ Support/AddressBook
With AddressBook working on the first machine it is time to setup the other machine or machines. Since you combined the AddressBook already, all you need to do is move the AddressBook directory from ~/Library/Application \Support and add a symbolic link to the Dropbox location. Again use the Terminal to issue these commands:
$ cd $ mkdir backups $ mv ~/Library/Application\ Support/AddressBook/ ~/backups $ ln -s ~/Dropbox/AddressBook/ ~/Library/Application\ Support/AddressBook
Repeat the Forth step for each machine you want to share your Address Book with.
Address Book wasn’t written with multiple copies accessing a single, shared set of data in mind. I haven’t experimented with having the Address Book application open on more than one machine at the same time. I would expect this to cause errors and possible data corruption. I rarely have Address Book open at all, usually only to update an address or add or remove people. If your usage paradigm is different than mine and you have Address Book open all the time, you may need to be prepared to restore from the backups you made.