Btw, in Buzz, you can unsub from a person's Twitter in-stream. RHS dropdown has "mute Twitter posts from X".
I routinely post anything I want to be publicly visible to Twitter. It's then forwarded to Facebook, LinkedIn and (soon) G+. If I want something visible only to my friends, I post it to FB (I keep the friend list there tidy) - hopefully I'll be able to forward those to my "Friends" and "Family" circles on G+.
I don't want to maintain yet another stream whenever there's a new social network thing going on. OTOH, I don't post my "stream of consciousness" to Tw, I try to use it as a broadcast medium. I am annoyed by "today I had X for lunch" type posts, be they on Tw, FB or G+.
I question too the utility of duplicating your posts twice. It's really hard to get it right without multiple overlapping groups of people seeing the same content twice. Plus the resulting fragmentation of discussion as well.
Is posting on two services really worse than posting only on one? At least people who are on one but not the other will get to see the post. Until G+ and FB make peace and inter-operate, this can't really be helped.