Spotify for iOS was updated yesterday. Among the features are not only bug fixes but also push notifications. When sombody subscribes to your playlists you get a push notification.

Feature list:

• New: Push notifications. Receive notifications when your subscribed playlists are updated, you get new subscribers, and more. You can choose which notifications to receive in Settings.
• New: Intro guide for new users.
• Fixed: Missing retina graphics on log in screen (iPad).
• Fixed: Retina album art is now always synced when you offline sync playlists.
• Fixed: App could sometimes become unresponsive after scrolling and navigating at the same time.
• Other improvements to Facebook login, screen locking when offline syncing, performance and stability.