Well 16:9 is pretty rare on anything but HDTVs; PCs are 16:10 after all. As for why 4:3 or 4:3-like has been the most popular, remember nearly all video content for these devices is SD, and SD content is most frequently 4:3 or 3:2 (the latter being NTSC DVD-like, which is what the iPhone uses)
I think 800x480 (15:9) is definitely getting more frequent in handhelds, in part because it's a pretty nice format for web browsing and having a keyboard (either on-screen via touch or as a slide-out horizontally rather than vertically) - when I said 4.8", TBH I was thinking of Samsung's WVGA OLED, although that's just because it's the only screen of that size I've ever really tried to get more info on (without much success):
http://www.ubergizmo.com/photos/2007/10/samsung-oled-roadmap_468.jpg