Not sure if anyones seen/done this or even knows about it, I was curious from playing with win8 so went hunting and found these links. I know you can do specific icons for iOS/android and it appears theres something better than favicon for windows 
http://www.buildmypinnedsite.com/ - windows 8 live tile 'generator'
http://www.buildmypinnedsite.com/windows7/en#step_7 - win 7 pinned icons with jump lists
And for reference - android and iOS
http://www.zen.co.uk/blog/setting-a-home-screen-icon-for-your-website-on-ios-and-android-devices/ or
http://osxdaily.com/2012/03/29/set-a-retina-ios-bookmark-icon/
http://www.buildmypinnedsite.com/ - windows 8 live tile 'generator'
http://www.buildmypinnedsite.com/windows7/en#step_7 - win 7 pinned icons with jump lists
And for reference - android and iOS
http://www.zen.co.uk/blog/setting-a-home-screen-icon-for-your-website-on-ios-and-android-devices/ or
http://osxdaily.com/2012/03/29/set-a-retina-ios-bookmark-icon/