Ars Technica: Siri is iPhone 4S-only today; where will it be tomorrow?
The whole thing’s decent, but for now, skip right down to the bottom of the article and read that section calling out the potential problems:
Michael Okuda also noted that voice input is generally inefficient. “Imagine I’m looking at some photos, and I want to say, ‘Up, up, left, down one, photo number 3362, no, the one on the left.’—that’s much slower than just clicking or tapping,” he said. “Natural language is, I think, going to have some significant limitations.”