We've spoken quite a bit before about the potential that Apple might one day "split the platform" with either an iPhone nano (very unlikely) or an iPhone HD (which we've been discussing quite a bit lately).
Right now, all existing iPhones and iPod touches sport 320x480 screens at 163dpi. The iPod nano, by contrast, was already at 202dpi during the previous, "fatty" generation. The BlackBerry Bold is at something around 217dpi. HTC is making 480x800 displays now like they're going out of style.