Accessible Dynamic Menus

Is it possible to have accessible dynamic menus, either the 'windows' 
style ones that drop down from a button at the top of the screen or a 
vertical expanding style menu that can expand downwards to display sub 
items.

I know that you can do some fun things now with lists and CSS, but can 
they constitute being accessible.

I would be interested to know what you think.

If it is possible, I would love some examples.

Cheers,

Julian Voelcker