Many of the other solutions will work, but they all make use of the open-toolchain for the iPhone SDK. So, yes, you can write software for the iPhone on other platforms... BUT...
Since you specify that you want your app to end up on the App Store, then, no, there's not really any way to do this. There's certainly no time effective way to do this. Even if you only value your own time at $20/hr, it will be far more efficient to buy a used intel Mac, and download the free SDK.