Shaun_Max yup, older versions of android don't even ask. On iOS, it's best to ask for that permission in a creative way. If they're about to do something, like say an action that starts a timer that they have to come back for (a la farmville), show something in your game's UI that says like "hey, we can send you a push notification to let you know when XXX happens".
If they say yes, show the dialog. Remember, on iOS, you get ONE chance to request these permissions. If they say no, iOS will NOT show it to them again until they reinstall the app. So don't just shove the request in their face on first app launch. Make it count.