For the past few days developers have been releasing updates to their iPhone apps that take advantage of some of the new iOS functionality, like multitasking. Some seem to be stating specifcally what new iOS 4 features they are taking advantage of, while others are just stating compatibility. Pandora is probably the best known app to use multitasking, a demo was shown at WWDC 2010, and it was released just before the iOS update was made available for download. Pandora uses the multitasking feature that allows an application to keep playing music even if the application is closed. The iPhone's iPod feature could always do this, but now any other music program can as well.
While Pandora is the best known app to be released so far with multitasking, I've noticed several others showing up; but strangely most are just stating that they have been "Optimized for iOS 4." Not sure what that means, but from some quick checking - Fast App Switching, not one of the optimizations.
Keep checking the updates section on your iPhone or through iTunes for apps that have been updated. If anything the updates should keep crashes to a minimum.