I've been having fun with Google plus this last week, while getting my head around the hybrid Facebook/Twitter structure of it. I found a great post by John Tropea on the differences between Google+ and Facebook/Twitter, and he sums up my initial confusion on how sharing with circles works. Essentially, when you post something to a circle, that whole circle won't see it in their stream unless they are all following you back, which is necessarily the case. I agree with John that this isn't very intuitive.
So, @Google, here's my simple suggestion to make it a little more intuitive: the little grey circle graphic next to each of your circles could represent what percentage of that circle follow you back - like a little circle graph. So if only half the people in one of your given circles follows you back then, then the circle outline is half red, half green (or some other colours), if 90% follow you back, then 90% of the little circle would look green, while a little 10% sliver would be red. I think this would be a quick, simple way for the user to see roughly how much of their intended audience might actually see their post.