notify_by Prowl do |prowl|
  prowl.on_success = true
  prowl.on_warning = true
  prowl.on_failure = true

  prowl.application = 'my_application'  # Example: Server Backup
  prowl.api_key     = 'my_api_key'

Prowl is an iOS push notification gateway. Backup can connect to Prowl and deliver success and/or failure notifications directly to your iOS device. All you need is a Prowl account. Go to the API keys tab after registration, generate a key and copy/paste it into your notifier configuration.