This is from Apple’s Snow Leopard info online:
While most companies store contact information in centralized LDAP directories, IT administrators usually prevent users from modifying or adding their own contacts to the directory. This makes it hard for users to maintain a centralized list of contacts and access those contacts on multiple computers and devices they own.
The new Address Book Server in Snow Leopard Server solves this problem. Not only does it store contacts on the server, but it allows you to access and use those contacts on each of your Mac computers. With Address Book, you can even synchronize contacts to your iPhone for accessing your contacts on the go.
Sadly, this isn’t the case. Address Book Server works beautifully between Macs. It’s fast and reliable. However, the iPhone OS doesn’t support CardDAV, the technology behind Address Book Server, meaning these contacts cannot be synced over the air to an iPhone like iCal or Mail data.
The line about syncing the iPhone isn’t accurate either. iTunes only can sync to the “On my Mac” collection of contacts. When hooked to a Snow Leopard Server running the Address Book service, it creates a new collection — outside the realm of iTunes’ (and Spotlight’s, curiously) reach. Basically, if you want your contacts across multiple Macs and an iPhone, Snow Leopard Server isn’t the solution you need, no matter how loud Apple sings it’s praises.
Which means I may end up renewing MobileMe yet again to get the features Snow Leopard Server ought to have.