This was politically potent. When a move was made toward, say, an open source data format like ODF, Microsoft could trot out advocates for the disabled to say how unfair this was to them.
No more. Or maybe not much more.
Ajax may seem like great stuff to the sighted, but how is a blind person to understand what is happening as they mouse over a word cloud? This has been a big problem for many years.
It took a huge effort to solve. The Alliance has over 100 members, including Microsoft. Solving the problems of Ajax is more important to all players than maintaining a monopoly on accessibility.
And this does not necessarily threaten Microsoft's dominance of clients, in the near term. If a blind person was looking to buy a PC today, I'd still recommend a Windows machine to them.
But the world is moving away from PCs. It's moving to Kindles and iPads and iPhones. Solving the Ajax problems on Windows Servers maintains Microsoft's momentum. The client difficulties are another subject.
OpenAjax has a number of very well-written white papers on the site, including this one on the basic technology, this one on browser technology, and this one on deployment strategy. OpenAjax is ready for prime time. It's a very good thing.