Out of curiosity, I just tried it on an account with an existing Google Profile that doesn't have buzz set up and it did not add followers/followings to the existing Google Profile. It added the status message to the buzz feed, which isn't a privacy problem since contacts can see it anyway, but it did not fully set up a buzz profile. Clicking on the Buzz status textarea brings up the normal "How do you want to appear to others?" form and still won't let me use Buzz without setting it up.

Also, I can't see the following/follower lists from another account; it just shows the name, a link to the google profile and the fact that the two accounts are following each other, but nothing about any other contacts. It does, however, appear that the profile is fully set up from within the account, showing the follower/following lists, but outside accounts can't see this info.

I also follow someone who hasn't created a Google profile or started using Buzz. She changed her status message and it showed up in the Buzz feed, but when I click on her name it goes to an empty Buzz profile view that says "This person does not have a public profile." where the name usually is.

TL/DR: I looked at accounts both with and without an existing Google Profile and although a status message change is posted to the Buzz feed, in neither case did it fully sign the account up for Buzz nor did it expose contacts, despite the fact that it looks like it does. YMMV, but probably not.