Android default browser on Android 3.0 and later (this is probably the biggest chunk of users)
Windows Phone 7
So, realistically, you're looking at people who still use Windows XP (unless they're using Firefox) and people with really old Android phones that'll never receive a manufacturer firmware update.
IE7+ (unless running on Windows XP)
Firefox 2.0 and later
Opera 8.0 and later
Chrome 6 and later (unless running on Windows XP)
Safari 3.0 and later
Android default browser on Android 3.0 and later (this is probably the biggest chunk of users)
Windows Phone 7
So, realistically, you're looking at people who still use Windows XP (unless they're using Firefox) and people with really old Android phones that'll never receive a manufacturer firmware update.