Locking it to the gmail account is a bad idea. It might make business sense for Google but for becoming a Facebook or Twitter killer, it is not with that limitation.
I presume they'll have a standalone Buzz application at the 2.0 or something similar. That will let an established gmail-only social graph generate lots of activity and use, seeming enticing to yahoo and hotmail (and all third-party email account providers) users that can now join/be invited in on the social-web-2.0-sharing fun.