The scripts look like they're organized by versions, ie ajax.googleapis.com/ajax/libs/prototype/1.6.0.2/prototype.js, so google can't really do any 'fixes' if they want to stick to the original authors' versions. Being afraid of have to 'depend' on this isn't a big problem; it's just a text file. You can create a constant in your codes for the URL string so that you can change it easily. The Script.aculo.us and other libraries are pretty big, so having it load from Google would help a little.
Maybe one possible solution to my concerns would be to keep a table of sha1 sums of the version you want to depend on, and check them periodically (randomly select 1 of every 1000 visits to receive the checking code that reports back to your backend).
I just have an instinctive wariness of this kind of dependency, even if the cost of avoidance is a performance penalty.
