Syncing address book contacts through iPad and iPhone?
There are a number of options, these solutions assume you want to sync your contacts to Apple's Contacts.app on your iOS device and not a third party app.

MobileMe

Apple's service is the most straight forward (it should just work) and offers other benefits such as storage.

Microsoft Exchange

iOS devices can use Microsoft's Exchange protocol to update the device's email, calendar and contacts. If you are running Snow Leopard you can use also connect your Address Book, iCal, and Mail to the same exchange server and your contacts will sync between your iOS device and your Mac's Address Book. A free option would be to use Google sync on your iOS device which uses Microsoft's exchange protocol. However Google Sync doesn't work for desktop applications like Address Book right now. So you would have to find a way to sync your Address Book contacts to Google which is possible.