I read "Web 2.0 & Accessibility for Disabled" posted by Kathryn Dove... I found the following checklist to be very helpful for Web 2.0 and people with disabilities:
Support accessible authoring practices
Generate standard mark-up
Support the creation of accessible content
Provide ways of checking and correcting inaccessible content
Integrate accessibility solutions into the overall "look and feel"
Promote accessibility in help and documentation
Ensure that the authoring tool is accessible to authors with disabilities

Presentation by Denis Boudreau which provides an overview (based on research) of the state of Accessibility in the social media sphere. A comparison of "accessible" features in common social media services such as Facebook and Twitter, an some pointers for the ideal future of the social Web with regards to Accessibility.

This website is a Web client for Twitter users that takes into account several features that make Twitter more accessible for users with disabilities. To name a few, it includes features such as: - "All links are keyboard accessible (and visual styling on focus as well as hover states). - Headings and page titles are implemented for optimal accessibility. This includes an H3 heading (for author screen name) added to tweets to assist with screen reader users. - Large default text size and high color contrast. - Text resizes many levels without breaking. Access keys (keyboard shortcuts) to main menu items."