You mean to use Xcode4? You can already use it by opening the old scaffold or demo project in Xcode4. They should be upgraded automatically. (I want to make new project files in the future, to make sure everything is done as Xcode4 expects it.)
If you mean it more generally: you can use many of Sparrow's classes in non-Sparrow projects! However, as soon as the class is displaying some content on the screen, it won't work, because the render methods are custom to Sparrow and won't work in another framework.
But e.g. the juggler and the tweens, and even the sound classes, should be usable anywhere else. Just be sure to copy not only those classes, but also their base classes (e.g. SPEventDispatcher) and classes they depend on (e.g. SPEvent).
I hope that helps!