I've had to do some research lately on a couple of projects and how well they comply with Section 508 laws. This has gotten to be a really big deal - Target just settled a lawsuit concerning the accessibility of their website for over $6 million. (Good summary here and full details here) That really got some of our clients' attention. Thankfully it's been something I've spent a lot of time thinking about, and our work is generally in pretty good shape.
Drupal has been the CMS of choice lately, and it's a very rich set of tools. Thankfully with a well-thought out theme it in general produces very clean HTML. We had already dealt with a lot of the basics (CSS-based design, em-based typography, hidden labels for navigation elements and pretty consistently semantic markup). I spent a few hours running pages through Cynthia Says and making some adjustments and now aside from one or two 'alt' tags for smaller icons we're about as close to 100% as you could like.
I think there's always room for improvement, but after surfing the primary site I've been focused on for a bit with CSS turned off, I'm pretty happy with the results. It's still a very usable site.
One thing about Drupal and the 'Image' module that I think needs some attention is how 'alt' tags are treated. It's fairly smart in how it will build up the tags if you are using the title and/or caption, but often an image is placed without the title being visible. The way you do that is by removing the title when inserting the image in the block or content area (so it stays in the actual image node details, but just is not displayed). Unfortunately this also means that the title is not there to be utilized in the 'alt' tag. I'm planning on proposing to the module maintainers that in addition to the option for 'title' and 'description' that they add in a text field for the 'alt' tag (prepopulating with the image title). This way the site will more likely stay compliant over time.
All in all though - have to say I'm pretty impressed by how easy it's been to finesse, having put some good thought into the basics from the beginning.