If like me you’ve got a work Exchange account, and another Exchange account somewhere else (or even a Gmail account which can use the same protocol) and sync these to your iPhone you’ll share my frustration in that Apple only allow you to sync a single account. Thankfully those days are numbered as now it’s possible to connect more than one account.
You’ll have to Jailbreak you phone if you want to do this, but it’s now pretty easy to add a second Exchange push email account to your iPhone.
First things first, Jailbreaking can be achieved fairly easily and safely using something like Blackra1n. Once Jailbroken you need Cydia (an Apt repositary/gui for the iPhone) installed and using Cydia, search for AddExchange and install.
After you’ve installed AddExchange, fire it up and press the button to add your second account. A second account should appear with some default settings ready for you to change to your own.
Now you’re up and running with the second account you can safely remove (via Cydia) AddExchange.
Another option is an app called MultiExchange which claims to do this in a much cleaner way – however it has to be purchased from the Cydia store for $0.99. As I’ve been happy with AddExchange I’ve not tried this but from what I’ve read it doesn’t suffer from a two account limit.
More details over at AppAdvice
[Update] iOS 4 now supports multipe Exchange accounts, by default.