The Help file for OS X 10.6.4 (Snow Leopard) Address Book sync to Google Contacts links to this oddly named Google page: Where can I find information about Contact Sync? - Contact Sync Help.
This is what we get.
I'm working up to trying this. Currently I sync my iPhone contacts like this:
- Corporate: to and from Exchange 2007 .
- iPhone Personal <-> MobileMe Contacts
There are two additional syncs
- MobileMe <-> OS X Address Book <-> Google Contacts (My Contacts)
The last of these is mediated by Spanning Sync.
Both MobileMe and Spanning Sync cost money. I could easily live without MobileMe Contacts.
So I'm considering either ...
- iPhone Personal <-> OS X Address Book <-> Google Contacts
- iPhone Personal <-> Google Contacts <-> OS X Address Book
Either way the Google Contacts to OS X Address Book would be mediated by Contact Sync.
Remember how easy this all way with the Palm III?
 This means my employer can wipe my iPhone at any time. This might or might not delete photos. Everything else is either synchronized or backed up, but a remote wipe would be a pain. Know your risks.