Twitter Instant Notifications - Twitter are launching push notifications for @mentions.

When you’re out and about, your phone probably isn’t always in front of you. It might be in your pocket or purse or on a table – perhaps with the screen off.

Whenever an account that you follow mentions you, you’ll immediately receive a notification. You’ll know who is talking to or about you on Twitter, and you’ll be able to continue the conversation in real-time. Notifications for @mentions will initially be available on SMS and on the latest version of Twitter for iPhone, which you can download today from the App Store.

http://itunes.apple.com/app/twitter/id333903271?mt=8

http://blog.twitter.com/2010/11/instant-notifications.html