What I like about the Google Web Accelerator is that it could be the start of a successful global caching system for media delivery.
What I dislike about the Google Web Accelerator is that it sees everything.
Whereas before Google was only able to read emails that were sent to or from people with GMail accounts (though of course you couldn't always tell who was using one, as often people would forward their mail to GMail from another account), now Google is able to read email accessed using any webmail account (except for those few that use https all the time).
Only one member of a private mailing list had to use GMail for all of the list's messages to be available to Google. Now it only takes one member of a private forum to be using the Web Accelerator for all of those private messages to be stored in Google's proxy cache, available on request to governmental agencies (presumably worldwide). Using the term Orwellian is such a cliché, but in this case people really are being used to watch each other's behaviour, even if unwittingly. You can't trust everyone not to use Google's proxy.
Blocking IP addresses on your server or using special HTTP headers is futile - the proxy IP addresses could change at any time, and the proxy cache is free to ignore headers if it chooses. The only solution is to use https for any possibly sensitive information that requires a login, encrypt your emails (and why not encrypt your IM messages as well for good measure).
Then just wait for the day when GMail offers to store people's private keys, to 'help' them read all their encrypted mail.