This is another 10.6, or an iPhone 3.0+, or maybe iTunes upgrade gotcha. Not sure which, since I have done all three upgrades since I last added a contact. A new contact was not syncing, so I started checking the forums for clues.
When I looked in iTunes, sync contacts was no longer selected, one of them unselected the sync contacts checkbox in iTunes. Quite a few other preferences have disappeared since I upgraded to 10.6, so I suspect that is the culprit.
I reselected it and it synced. But it found two mismatches between the Address Book and the iPhone. Apple Computer Inc. does not match Apple Computer Inc. and another I cannot remember, also a perfect match. All entries predate the upgrades and I haven't seen the mismatch message for 2 years.
I forgot about iPhone search so I didn't try that. Hope that helps someone.