<<The section below requires enabling Google Sync in the Mobile section of your domain's Google Apps Control Panel..>>

Get Mail, Contacts, and Calendar on your iPhone
Set up Google Sync on an iPhone	Set up your iPhone with Google Sync to access your Google Apps mail, contacts and calendars using your phone's native applications.
Google Sync Help Center	Find out more options and features of using Google Sync on your iPhone.

Use Google Talk on your iPhone
Point your iPhone's browser to: m.google.com/talk	Then chat with your Google Talk contacts directly from your iPhone. More about Google Talk on the iPhone

Get Google Docs on your iPhone
Point your iPhone's browser to http://docs.google.com/a/your-domain.com/m	From here, you'll see a mobile version of your Docs list where you can view documents, spreadsheets, and presentations. More about Google Docs mobile

What other Google Apps can you use on your iPhone? Visit Google's iPhone Help Center.