Icons are a very important aspect of a mobile OS. You see them all the time. And Nokia has treated us with what I believe is one of the most fine icon sets of all time with the N9.
While I admit that I like borderless/backgroundless icons (N900, Android) most app developers have no icon-making skills and make really ugly icons, especially in the FOSS world where no big corporations with design department do apps.
So when your app grid gets filled with 3rd party apps, having a background makes the grid as a whole look much more consistent and tidy.
That’s why apple auto-composites icons to it’s own background. So that dev’s don’t even have to bother with making an icon look like the others. They just choose an image and it does the compositing (there’s of course the option to disable that and create your own composited icon).
My idea is to create a dominating shape for all icons which is not a hard background like N9 but just a guideline for all OS apps. The icon is allowed to deviate a bit from that shape with things sticking out or negative space eating in.
For example look at the settings icon of FirefoxOS
This gives freedom to designers to create something special but also allows developers to use a generator that slaps any icon of any shape on a circle (or crop it to a circle) and create an icon that doesn’t look out-of-place.
Hurrian’s icons use this approach, and I am readying an icon set myself that adheres to the same guidelines.