If you plug an MHL adapter into the Galaxy Nexus, it will rotate the screen and keep everything in-place.

Third party launchers usually allow you to rotate the screen, but I can't say for certain if a stock launcher since 1.6 on the G1 ever did.

Most Android tablets up to this point seem to prefer landscape orientation. The inability for the Nexus 7 to operate like that is just a little strange to me. Not a deal-breaker.

EDIT: Here's a youtube video showing the Galaxy Nexus over MHL. You can see that the Nexus is just mirroring what's shown on the monitor.