Navigation bars with dynamic content

Hello all,
I am sorry if this question has been already asked on the list, but if 
someone can point me to the archives dealing with it, it would be nice.

We encounter more and more web sites that use dynamic navigation bars with 
submenus appearing when you move the mouse on them.

People who are responsible for these web sites argue that these dynamic 
menus are useful because they appear when you need them. But in many cases, 
those menus are not accessible with a keyboard.

Does anyone know examples of such menus that are accessible?

An example of inaccessible navigation  bars can be found at:
www.francetelecom.fr/vfrance/home/pourvous/index.htm

Thank you in advance for your help

Regards
Sylvie