I will say about layout that I think a lot of companies need to think really, really hard before they introduce something that is such a departure from the existing as the ribbon is. I can't honestly comment whether Microsoft has, on the one hand you would think they'd done a lot of user research but on the other hand it's possible that pressure from above could have made them glaze over this. I mean you'd think if there were doubts over a space shuttle launch they would give it a long hard look before giving the green light, then we had a disaster like Columbia. If Nasa can do that when lives are at stake then it is possible for a company to do the same. I'm not saying they've definitely made this mistake, just that we have to consider the possibility.

As to "overlay interfaces" I'm pretty sure most screen readers use that kind of interface for the web as a whole, putting the web page into some kind of buffer. So long as it's only there to reinterpret the content I don't see it being an issue, just like how you might make an alternative front end for a database. It's when they start building their own interface from scratch that we have issues about closed systems. I'm not saying they should do this for an entire application, accessibility should be based on having as little messing around like that as you can get away with for the sake of efficiency and should be focussed on specific controls as tightly as they can make it.