Yep I worry about it too. I pretty much feel like an awful parent and wonder just what bad influences they are picking up off me. My 3 year old seems extremely confident though. She has no fear of walking up to strangers and handing them toys, or playing with other children. She has her problems (she still isn't talking very well and she won't potty train at all, she's constantly on the go and getting into trouble, lots of extreme temper tantrums) but I don't think she's SA or about to become so. I think my husband might be a good influence there too. He's very extrovert - he doesn't understand at all what I'm trying to tell him about how I feel, but at least my children have him as an influence too.
My 8 year old, well I have worried about her occasionally. I don't think she's SA I think it's something else, because she doesn't seem to mind drawing attention to herself in social situations, even when that's bad attention, whereas I'd rather shrivel up and die than be noticed at all! For instance, we took her to the dentist this morning. I was feeling bad that she was picking up SA from me because she was crying and being sad, panicking and generally trying to get out of going, even though what she was going for was something she had had done before and wasn't painful and she knew that. However, I think it's something different than SA because when she got there she threw a huge tantrum, wouldn't let them do the work and in the end they gave up and we left. I think if she was really SA at all she would have sat there and let them do the work rather than kick up a fuss!
(Incidentally I plucked up the courage to go too, with my daughters and my husband - wish I hadn't. The dentist at one point ordered me out of the room really rudely cos 'she was worse since I got here' and I've felt awful ever since)