The goal here is not to find out if an e-mail address is valid. It's to find out if the guy that leaked inside information on an Apple fansite is an Apple employee. It's an issue of privacy, not spam.
Maybe if the apple employee doesn't want to get caught he shouldn't use his apple.com email address that has his real photo as his gravatar.
"Who leaked this info?" "I don't know but it's someone who looks just like Bill! They even use the same gravatar image he uses everywhere else! Devious bastards! Let's see if we can reverse engineer this email hash to find out who this rogue might be!"
Well of course, my example is an exaggeration, but the idea is that this exposes the poster without them knowing that they are exposed. At the same time, so do a million other things like your IP address in Google's logs on the way to the comment you made on that blog. Let's face it: there's no privacy on the Internet. There's just varying degrees of completely exposed to sort of in the shadows.
Well my point was that this isn't really a problem caused by hashed email addresses as much as it is a problem with using email addresses in general. Even if Gravatar used something to completely hide your email address - if you use the same email address in different places your gravatar will be the same and you've compromised yourself, regardless of how they stored them.
