Just found this out the other day. My employer uses Google Apps for our domain - Mail, Contacts, Calendar, Sites, all that good stuff.
If your administrator turns on Google Sync for the domain and you have iPhone version 3 or above, you can configure a Microsoft Exchange mail account according to these instructions.
With this turned on, you get push mail, contacts and calendars. It's fun to have your calendar open, go to the web, make an appointment and see it appear in seconds on the iPhone.
I was a little anxious about what would happen to my existing contacts, sync'd with my Mac, but all is well. The contacts from my company's list appear as a separate group and further I get a "Global Address List" group as well.
It's funny how Google never once mention the word "Exchange" and try to refer to this protocol as Google Sync.
Note that you can only have a single Exchange account set up on a phone.