What I am guessing is that it will come in the same as an app update from the market. Essentially the market was made smart with the recent release of HC, to be able to detect the device that is signing in with your google account. Its software version is listed as well (which is why some people with custom roms cant send apps to their phones via the web marketplace) so the market will be able to tell if there is an update, notify you... and then give you the option to download it. They cannot push an update to your Xoom without a wifi connection, and due to it being intermittent, you can guess that it will be optional.