I have run into situations where I may have to maintain Address Book entries between multiple systems. Here is how I do it using a MacBook Pro, an iMac, an iPhone, a Blackberry, and a Windows PC.
Assumptions/prerequisites:
Gearing up
On the Mac that has all your contacts (we will call it Mothership), back up your Address Book. You can do this by opening Address Book and going to File » Export » Address Book Archive. Save this in a safe place. Then create a Group VCard. To do this, Select All Contacts within Address Book and then choose File » Export » Export Group VCard. Save this VCard to your desktop.
Cleaning
Now, delete all contacts from all of your systems, including the Mothership. This would include contacts on your Windows PC (Outlook or Address Book or Contacts), your Blackberry (you would need to use the Blackberry Desktop Manager to do this), your Gmail account, your Mobile Me, and any other system that you wish to sync with.
Configuring
On your Macs, configure the MobileMe preferences (within System Preferences) to sync contacts (apart from anything else that you may want). This can be done within the Sync Tab.
Syncing Address Book with Google: This can be done by opening Address Book on your Macs, navigating to Preferences » Accounts, and checking the 'Synchronize with Google' box. You will also need to click on Agree on the drop-down agreement, and then enter your GMail username and password. Note: Google sync only happens once every hour. To change this, you need to change the Start Interval parameter in the com.google.GoogleContactSyncAgent.plist file found in ~/Library/LaunchAgents folder.
Repeat these steps on all your Macs, then sync your iPhone Contacts with Mobile Me. To do this, follow these instructions for either your Mac or your PC.
To get your Blackberry into this sync circle, download Google Sync utility on your Blackberry. To do this, go to Google mobile on your Blackberry, and download the Sync application. Run this Google Sync application (which will require you to log into Google using your Gmail username and password). Set up the Sync Contacts section to sync automatically.
The final part
On your Mothership Mac, log into GMail and navigate to the Contacts section. In here, click on the Import button (usually on the upper right corner), and then on the Choose File button. Navigate to the Desktop and select the Group VCard you exported from Address Book. Complete the import process by clicking the Choose button. This may take some time, depending on the size of your Address Book VCard and your bandwidth.
Testing
You could manually start the sync on your Macs using the Sync Now option in the Sync icon in your menu bar. Your synchronization should now start with all entities in this order: Google » Blackberry and Macs » MobileMe » PC and iPhones. The sync order will depend on the time when each various sync process starts up. Once everything is in order, any change in any one entity will sync all others.
[crarko adds: I haven't tested this one.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2010050409012935