Hey, kids, heard the news? Jailbreaking your iPhone is legal! The US gummint sez so!
And now that it IS legal, you can play with all kinds of nifty, non-Apple-sanctioned apps. Like, f'rinstance, the new i4-ready version of My3G, which bamboozles your iPhone to think you're on a Wi-Fi connection instead of that data plan you're (over) paying for. So stream those high-def YouTube videos, save your voice plan minutes by VOIP'ing away, and — best of all — FaceTime chat to your heart's content without dashing to the nearest Wi-Fi hotspot. All for US$3.99.
(What? You haven't jailbroken your i4 yet? With JailbreakMe out there, complete with our step-by-step instructions on how to use it? Are you kidding? Go! Now!)
BTW, developers of My3G have MyWi app which we covered earlier.