With the release of iOS 4, you can have multiple Exchange accounts set up to push Mail, Contacts and Calendars to your iPhone.
I already have my personal email set up as an Exchange account on my iPhone, which works great, however, at work, we use Google Apps for Mail (and calendars etc..)
Currently, I have to use iCal to get my work calendar from Google Apps shown on the iPhone, and there’s no way of syncing my work contacts, as well as my personal contacts from my hosted Exchange account.
Since Google Sync on the iPhone takes advantage of the Exchange protocol, we weren’t able to use both at the same time, until iOS .
This is great news, as it means I can now have both my Google Apps, and hosted Exchange accounts syncing properly on my iPhone.