Well, this is interesting. Google has temporarily suspended downloads of the accelerator, citing capacity issues:

Thank you for your interest in Google Web Accelerator. We have currently reached our maximum capacity of users and are actively working to increase the number of users we can support.

There's some speculation that it might be part of a deeper problem, with this being a face saving excuse. Have a look at this:

The most disturbing aspect is Google Accelerator could prove to be a serious security problem for web sites specializing in dynamic user-generated content, such as forums and other community sites. This has always been an inherent problem when using carelessly configured caching proxy servers, but never before has such an expansive and intrusive caching platform been offered for free, thus the problems are likely to get much worse before improving as thousands more unsuspecting people install the Google Accelerator client each day.

Try using Web Accelerator on a forum site, one with lots of geeks who love Google and probably already have Web Accelerator installed. Why, if you're lucky, you'll be logged in as someone else, as the folks at SomethingAwful.com discovered. The posters in that forum discovered that most of the times they refreshed the page, they were logged in as a different person, seeing their friend's control panel for the forums.

Looks like there may be problems with the aggressive assumption that everyone follows the spec recommendations on GET completely. The thing to remember about this is that there are standards, and then there's actual practice. The latter tends to be a lot more relevant.