The Twitter iPhone app now comes with a clever little feature: notifications for @mentions. It means that whenever someone you follow mentions you on Twitter, you can receive either an SMS - as you might receive notices of Direct Messages - or a pop-up notice if you're using the latest version of the Twitter iPhone app.
The push notifications will require the latest version of the Twitter app which is available now. [Twitter]
Update: Turns out that the app allows you to set whether you want to see @mentions from only folks you follow or from everyone. Very clever.