Has user testing found that the "three horizontal bars" for main menu on mobile is commonly understood?

"Three horizontal lines" aka "hamburger icon" seems to be becoming a convention for the "main menu" button, especially on mobile sites:

Has anyone done any user testing of this convention, or got any A/B testing results? (i.e. is there an increase in bounce rate compared to using the word "Menu"?)

I did some quick user testing with wireframes featuring this icon for the main menu. 3 out of 6 people got it and used it. The others did not.

One person in particular had a lot of trouble "getting back to the top" (i.e. restarting their navigation from the top level of the site) as they didn't know about the convention, or that the logo would link to the home page.

I guess it will be different for different demographics - users of Facebook on mobile probably know it.

Anyone got any findings they can share?