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.
One big thing for me with regards to the iPhone Microsoft Exchange integration is the inability to create and manage folders.
I am one of those people that religiously file emails away once dealt with – for example, I have an "eBay" folder, with "buying" and "selling" sub folders for all the emails relating to my purchases and sales.
The iPhone (unlike Windows mobile devices) has no ability to create, delete or rename folders on the Exchange server, which is a bit of a shame.