In a recent project I needed to have my secondary links display on the home page, but for some reason they weren't displaying. I had the source for the secondary menu set in the menu settings to be the 'main menu' and my 'main menu' was setup like this:
HomeSub 1Sub 2Nav 1etc.
My 'Home' link's href was set to <front> and the secondary links worked perfectly, except on the home page.
After some googling, I came across this URL: http://data.agaric.com/get-secondary-links-show-under-front-home-page-drupal-5, and decided to change my home link's href from: '<front>' to 'node'. Sure enough, everything worked perfectly.