Case law has been the most helpful in illuminating the implications of the ADA for websites.There have been lawsuits involving companies like Expedia, Hotels.com, Southwest Airlines, and Target as defendants and primarily featuring accessibility organizations as plaintiffs. These cases had mixed results, but each helped clarify the ADA's jurisdiction on the web.
I have previous experience with a company that sold a platform to help alleviate ADA compliance issues on websites. From my understanding litigation is being brought to a few industries with it beginning to trickle down into others. Government agencies, education, banking, and law firms were some of the early initiators that were paying more attention to ADA before any lawsuits approached their door.