mouse-over menus

Hi Everyone- I work at the University of Kentucky College of Arts & Sciences and we are currently in the process of redesigning/rebuilding our website using Drupal 7. One of the features we're planning on using is the mouse-over expanding navigation (something similar to: wh.gov or vanderbilt.edu). I've been looking all over the ADA and WPI sites trying to figure out if this breaks accessibility rules in general, and for braille readers in specific. Any help on this will be greatly appreciated. Thank you.