I was thinking of all of you today, (I started reading this thread last night) while driving with all 4 of my children strapped in their car seats...but our now outgrown infant seat in the front passenger seat as we drove to see if we could "cash it in" at the local OM store for a different, more comfy stroller. I was horrified someone would see that seat, sitting sideways no less, and think there was a baby in it and call the cops on me! Of course, there would be no issue because the seat is empty...but you all have made me think.
For me, the bottom line is that with carseats, there is law. If you see someone breaking a law, do you choose to get involved and call the authorities? For the example of birthing decisions there is no "law" and so we are unable to judge what is legally appropriate. It comes down to differences in beliefs and parenting. Car seat laws supercede any differences in parenting philosphies, and IMHO you'd have to be living under a rock in this country (US) to not know your kid has to be in a seat. So I won't ever accept an ignorance excuse. The only excuse people can claim (but usually don't) is stupidity.
As for the original question...I honestly don't know if I would call the cops. I would probably do that over confronting the person, say in a parking lot or something, but that's just because I'm too afraid someone will totally go bonkers on me for poking my nose in. Chances are they know they are doing something wrong and in my experience guilty people usually aren't very nice when they get called on it.