iOS doesn’t use a custom font, it uses Helvetica Neue, Helvetica, and Avenir. There’s no reason other OSes can’t just adopt an existing font to use system-wide. For instance, Futura is still up for grabs.

Another font that isn’t used anymore, but which was custom designed for a WebOS, is Font Bureau’s Prelude. I like that one a lot better than the choices all of the other platforms have made: http://palmwebos.org/wp-content/uploads/2009/07/webos-font-p...