Most notably the update revamps the webmail system, and replaces the row of icons at the top of the window with a new application switcher interface. You can read about in Apple’s documentation.
Apple has also outlined the new webmail features, and you can read about them here.
Of course, if you are a subscriber of the service, you use the “Find My iPhone” application if you ever lose your device. Apple has now released a iPhone app that brings that functionality to both iPhone and iPad.
If you lose your iPhone or iPad while on the go, simply install this free app on any other iPhone, iPad, or iPod touch to find it. Sign in with your member name and password (MobileMe subscription required) to locate your missing device on a map and have it display a message or play a sound. You can even remotely lock the device to protect your privacy, or wipe it to permanently delete all of your data if you think that it won’t be returned.
I still insist that Apple should either offer this service for a lot less or for free, or they should significantly improve it with new features that could knock our socks off. Instead, Apple overcharges for a service that more or less boils down to wireless syncing of contacts and calendars.