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The coverage of web accessibility (BBC News / W3 WAI response) raises some interesting points. Disabled access is important, but there are other reasons to make a site accessible that have revenue impact: accessibility usually delivers a better customer experience; pages tend to load faster; they are easier for search engines to index leading to better rankings; etc. etc.
Advertising revenue will become another plus. This piece in Google’s AdSense FAQ (branded ‘AdWords’ to advertisers) caught my eye:

2. How do I optimize my site for the most relevant ads?
Our ability to target ads to your site depends on the content and structure of your site. Here are some basic guidelines for optimizing your site:
- Place ads on pages that predominately contain text — only text is used to determine a page’s context.
- If you have a robots.txt file, you’ll need to remove it or add the following two lines to your robots.txt to allow our content bot to crawl your site:
User-agent: Mediapartners-Google*
Disallow:
- If your page contains frames, select the ‘Framed page’ checkbox when generating the ad layout code for that page.
- Place ads on pages that don’t require a login.
- Place ads on content pages that don’t change frequently.
By following these tips, we can better serve the most relevant AdWords ads on all of your content pages. If we are unable to crawl or understand the content on your site, we may serve public service ads or your specified Alternate Ads, for which you will not accrue any AdSense earnings.

So, the bottom line is:
Accessibility to Google’s indexing robots = Greater contextual ad revenues