We have something called the disability discrimination act here in the UK, which pretty much rules out many interesting uses of Javascript if things like screen readers can't process them, and if there's no other way of providing that enhanced functionality to disabled users.
As others have commented here, I'm not convinced that the Google feature is in fact much more than eye-candy; and thus, since it doesn't really add any functionality, isn't really covered by the DDA. However, as soon as it actually becomes useful for something, then it will be covered; and I don't fancy the job of getting JAWS or something like that to interpret the JS in a meaningful way!