Last time I discussed accessibility here, it was to hold out a carrot. Designing for accessibility makes your site more mashable, skinnable, and maintainable. But if the carrot isn't enough, the government holds a big stick.
The U.S. District Court of Northern California has just ruled that a class-action lawsuit can proceed alleging that Target failed to make its web site accessible to the blind. Every case is unique, and I can't speculate on what the outcome will be in this case, but it sends a strong message. Accessibility is the law.
The regulations squeeze from two directions at once, like the jaws of a vice. Not only may sites be required to comply with Americans With Disabilities Act as in the case above, but the federal government (the biggest purchaser of technology in the world) will only do business with vendors who comply with accessibility guidelines.
While many popular web-development tools (like Internet Explorer Developer Toolbar) recommend that you check your web site against Section 508 guidelines, it's questionable if simply validating Section 508 is enough. That's the problem with validators -- any creative person can conceive of a site design which is accessible yet doesn't validate, or a site design which validates yet is not accessible. You should use 508 only as a starting point.
Another challenge is that the legislative guidelines were developed before AJAX and RIA really took hold, and the standards bodies are only just beginning to catch up. It may be some time before the legislation catches up with current practice. But this shouldn't deter you from taking accessibility seriously. Several countries besides the U.S. have regulations regarding web accessibility, and these regulations will only become crisper over time.
And besides, when the carrots are so tasty, why get too close to the stick?