I would not mind some sort of 16:9 mode. I know it's a challenge (read: extra work for Apple) but it's also a reality that we are living in a 16:9 world now when it comes to virtually any display over 14". Every user interface for such displays has had to adapt. The iPad is somewhat unique: a "post-PC" device with a very PC 4:3 screen. I like this screen, very much so, but bottom line is that when you redirect your display to another, larger screen, that larger screen's aspect ratio is now what matters.
If I take my 16:9 or 16:10 laptop to any conference room at my business and connect it to the fairly typical 4:3 projectors that many conference rooms have, I expect a 16:9 display on the big screen and black bars on the side on my laptop screen. If my laptop had a full screen but everyone else in the room saw letterbars, it would look ridiculous. I'd like to see the same adjustable UI on the iPad.