With the upcoming release of iOS 7 you will notice that the "standard" icon radius has been increased. So try to do what Apple and I suggested with this answer.
It appears that for a 120px icon the formula that best represents its shape on iOS 7 is the following superellipse:
|x/120|^5 + |y/120|^5 = 1
Obviously you can change the 120 number with the desired icon size to get the corresponding function.
Original
You should provide an image that has 90° corners (it’s important to avoid cropping the corners of your icon—iOS does that for you when it applies the corner-rounding mask) (Apple Documentation)
The best approach is not rounding the corners of your icons at all. If you set your icon as a square icon, iOS will automatically overlay the icon with a predefined mask that will set the appropriate rounded corners.
If you manually set rounded corners for your icons, they will probably look broken in this or that device, because the rounding mask happens to slightly change from an iOS version to another. Sometimes your icons will be slightly larger, sometimes (sigh) slightly smaller. Using a square icon will free you from this burden, and you will be sure to have an always up-to-date and good looking icon for your app.
This approach is valid for each icon size (iPhone/iPod/iPad/retina), and also for the iTunes artwork. I followed this approach a couple of times, and if you want I can post you a link to an app that uses native square icons.
Edit
To better understand this answer, please refer to the official Apple documentation about iOS icons. In this page it is clearly stated that a square icon will automatically get these things when displayed on an iOS device:
So, you can achieve whatever effect you want just drawing a plain square icon and filling content in it. The final corner radius will be something similar to what the other answers here are saying, but this will never be guaranteed, since those numbers are not part of the official Apple documentation on iOS. They ask you to draw square icons, so ... why not?