$main_menu empty after upgrade from 6

I upgraded my D6 site (acquia distro) to 7 beta 1 and I can't seem to get the main menu to display.

Main menu is set to display but the $main_menu array is empty. I tried adding the line described here:

http://drupal.org/node/784996#comment-2954338

...but it's still empty. So I thought I'd post here whilst I hunt around in case anyone has an idea of what it might be...