iOS 5 features: Wi-Fi Sync (Updated)
One feature of iOS 5 that has become a favorite during my beta-testing of the new mobile operating system is Wi-Fi Sync. This is exactly what a lot of iPhone users have wanted since 2007, and now it's finally here and working well.
To sync with iTunes on your Mac over Wi-Fi, you need to open Settings > General > iTunes Wi-Fi Sync on your iOS device and also have iTunes up and running on your Mac. On iTunes, be sure to check "Sync with this [iOS device] over Wi-Fi," which is found in the Summary information for the iOS device you're trying to sync.
Note that your iOS device must be plugged in and connected to Wi-Fi for the sync to take place -- it won't sync while on internal battery power. However, I was able to fake being plugged in by switching on my Mophie Juice Pack Air so that my iPhone thought it was plugged in. Correction: Your device will sync while unplugged, as long as it has enough juice onboard. What it won't do without external power is back up to iCloud wirelessly -- thanks to our commenters for setting us straight.
So, what gets backed up to iTunes? Just about everything. On my iPhone, I have ringtones, music, movies, podcasts, iTunes U content, and apps, so all of those items sync automatically. Whenever I update an app on the iPhone, the new version is synced to iTunes automatically.
Speed-wise, the initial backup (if you're doing a complete backup) or sync goes pretty quickly. For me, it seemed like the syncs were going much faster than they did over USB, although that might be an artifact of other improvements in iOS 5 instead of the network being used to transfer the data.
I've taken to running iTunes as a full-screen app in its own Lion space just so that my devices have more of an opportunity to sync with the Mac over Wi-Fi. Wi-Fi syncing makes syncing up your iOS devices incredibly pain-free. You can also choose to back up your device to your computer over Wi-Fi, once again selecting that option on the iTunes Summary screen for your device. You can choose to back up to your Mac or to iCloud, but not both.
Between the powerful iCloud syncing capabilities of iOS 5 and Wi-Fi syncing of your content to your computer, Apple is doing a great job of insuring that your data is safe.