On most of my sites I use an image map for the top menu navigation. The image has a usemap="#foo" tag and I leave the alt tag empty. Instead I use alt tags in all the area's that have been defined in the map. This works good in graphical browsers like IE and FF.
I use these image maps because one image with multiple click areas loads faster than many separate small images for each menu choice. I also never had problems with search engines spidering them. A third nice feature is that when you visit the site with a text browser like Lynx, instead of a large list to all top-menu links, there is only one link to the separate main menu list of links to the other pages. Therefore it takes just a few keystrokes to reach the actual content.
So far so good, but I have no idea how image maps are viewed by people with braille readers and comparable devices. Are they easy to navigate or is it better to go back to the one image-one-link approach to enhance accesibility?
@lammert: I see no problems as long you use the alt-attribute to provide text alternatives. WCAG checkpoint 1.1. In the <img>-tag it should read something like "Navigation links". In <area>-tag it should of course be related to the page the link points to, like "About lammert".
"Dive Into Accessibility" does not object to image maps either.
These are client side maps so that is no problem. I will take RonPK's advice to add an extra alt text for the map itself. I only had the alt for the areas because that is what people with a visual browser will see when they hoover over the areas, but I understand the use of the map alt text.