Turning off "push." "Push" email, contacts, and calendar means that your iPhone is updated instantly, automatically, wirelessly, whenever changes are made to your email, address book, or calendar on the Web or one of your computers.

"Push" means never having to check for updates, but it also means faster battery drain. At Settings→Fetch New Data, you can turn the "push" feature off to save battery life when you're desperate. Now your iPhone checks for updates only on a schedule—every 15 minutes, every hour, or whatever you specify in the Fetch section here. (Except for Web bookmarks. They're always "pushed," no matter what your setting here.)

Or, perhaps wisest of all, you can choose Manually. Now the iPhone updates the data in each program—Mail, Calendar, or Contacts—only when you open that program.

Conflict Resolution. What happens when you change an address book entry on one machine, and someone makes a different change to the same person's entry on another machine? How does MobileMe decide which change "wins"?

Easy: It asks you. On your computer, a message alerts you to the conflict, and offers you the chance to pick a "winner" right now, by comparing the conflicting entries and clicking the one you want.

MobileMe and Exchange. If your iPhone connects to your company's Exchange server, as described in Chapter 15, then you may have noticed, to your alarm, that you can't keep any personal data on your iPhone. That is, your company's address book, calendar, and email information replace your own address book, calendar, and email data. The iPhone can fill itself with only one or the other.

Or not. Here's a little-known perk of the MobileMe service: It permits your iPhone to display both your corporate Exchange data and your MobileMe data side-by-side. Exchange + MobileMe explains all.