Oh, thanks! I can't believe I never thought of poking around in the accessibility features to look for TTS. What I used instead on my iPod Touch was a dedicated TTS app (whose name I can't remember right now): to hear an Instapaper article, for example, I'd copy the entire thing to the clipboard (which was a little inconvenient since there didn't seem to be a "select all" command) and then paste it in to the TTS app. If I ever go back to iOS, I'll be sure to remember this tip.