You can start out with the CRM Outlook client - with that would will be able to sync your CRM contacts to Outlook. From there you can set the email account on your iPhone to also sync contacts down to your phone. With this setup, a change in either CRM, Outlook, or the phone should eventually be reflected back to CRM (assuming the contact is in CRM and not a "personal" contact in Outlook.
You also have the option of using the native mobile site in CRM - just by browsing the the CRM site from your phone you should get the mobile view (assuming contacts are enabled for mobile).
You can also look into third party apps like CWR Mobility and Resco for additional mobile functionality.