It doesn't look like it shows the actual content of e-mails unless you search for something like "confirmation gmail". "confirmation" just gets you the "Messages from..." box which you have to click to open.
Non-tech users (probably 95% of Googles customers) have no idea how this happens. It's not immediately obvious to a non-tech person how your personal e-mails suddenly get blended into search results on another page. This will both freak people out and it will invade their privacy.
I really hope this will be opt-in.