Bruce Lawson, a web accessibility expert, certainly thinks so. Lawson writes on the subject for the May 2008 edition of the Internet Newsletter for Lawyers. INfL is a subscription based publication (online and offline), so you'll need to be a member to read the full article.
Lawson's argument about Sharepoint websites breaching the Disability Discrimination Act (DDA) breach is based on testing which indicates that such websites breach a number of the Priority 2 criteria of the Web Content Accesibiity Guidelines 1.0 (WCAG). The link between WCAG compliance and the DDA is one that hasn't been discussed by the UK court. Many UK IT lawyers are therefore likely to find Lawson's argument logical, but not necessarily persuasive simply because of lack of case law. In other jurisdictions, the law is more settled because of Court rulings (e.g. see our report on the Californian ruling on Target).
You can read more about the DDA and website accessibility in our IMPACT article "Making websites accessible to all".