I've discovered that newer versions of the Android OS support Apple's touch icon format.
rel='apple-touch-icon-precomposed' are recognised.
Rooting around in the developer reference confirms this; there's a method in Android's WebChrome Client class to handle touch icons.
I haven't had a chance to play with this on an actual Android device yet! Perhaps I should go and do some window shopping :-)
This doesn't require much action, except perhaps some documentation.