Wi-Fi Sync is now available to purchase for $9.99 — but you won't find it on the iTunes App Store. To get it, all you need to do is download and run the Spirit jailbreak (click here), which can be used on any iPhone or iPod touch. It takes less than a minute, and frees your phone from Apple's control, unlocking a world of 'unofficial' apps that have been rejected from the App Store - including Wi-Fi Sync - and it's completely free, safe, legal, and reversible.
Once that's done, you'll see the Cydia app on your device. Cydia is a third-party app store where Wi-Fi Sync and many other innovative apps are available to download.