On MacOS X, you can write a Spotlight Importer so that your documents and content can be indexed and searched with Spotlight. I would like to write an iPhone app that generates content that can be searched with iPhone 3.0's new Spotlight search feature. Is this possible, or is this only available on MacOS X?
No, Apple does not allow third-party developers to write Spotlight plugins for the iPhone at the moment. You should file a feature request with Apple if you want this feature.