I have used Gmail since you needed an invite to use it and I've never had any other assumption than my e-mails being about as safely tucked away as they would be on a public notice board in a town square, and that Google in addition to this is probably scanning them frenetically to figure out what they, or more specifically their advertisers, can try to sell me.

If I need to send something I actually care about I wouldn't use e-mail in the first place. It's a low-effort type of communication with normal people who don't care about security, so I'm not going to waste time making my end of that line of communication an impenetrable bastion.