Up until now it wasn’t convenient finding those apps that are optimized for iOS 4, which is why Apple created a new category in the App Store called iOS 4.0.
This category includes all apps than can work in the background and/or apps whose graphics have been improved for the new iPhone 4 Retina Display.
You can access the new iOS 4.0 category in the App Store from here.
[Thanks Justin for the tip]