A binary DOM would be much more efficient than HTML, by an order of magnitude or more. When I was working on the BlackBerry browser we did the same thing and had gains at least as big. The problem comes when you have document.write calls in scripts, and start getting dynamic content inserted during parsing. That gets really tricky to handle in a server-side environment if it depends on client-side properties or user input.
Very true - this is what old WML based devices use to do - WML got compiled to WBXML or WMLC (depending on who wrote the docs :), and that got sent to the device, making much easier to handle parsing for the device. There were really big speed ups both computing wise - but (especially in those days) also transport.
I don't know how Amazon is doing it with Silk, but I'm guessing they're seeing enough of a speed up to make it worthwhile.
