We contacted Fitterer to find out exactly what he meant by this -- he explained, "Apps can play songs, but they can't get at a song's bits (needed to analyze it and build a matching rollercoaster + traffic pattern)." He added that there were workarounds -- for instance, a player could upload a song to the App or through a central server, which would create a level for them. However, he said that system felt inconvenient, and that he doesn't "want to put Audiosurf on iPhone until it can really let you ride your music - all of it."
There's still a chance the game could end up on an Apple product -- though Fitterer negotiated directly with Microsoft to get his game on the Zune HD platform, the two parties came to no form of exclusivity agreement. "I'm ready for Apple to call," Fitterer joked.