I'm too lazy to read through this whole thread right now, but yeah, I support some kind of restriction. A test would be AWESOME. And of course, if you fail, you'd be allowed to study up, fix up your life and take it again later.
Or if that's too extreme, (I know some people would go batsh*t, even though all it really means is that they'd have to put a reasonable amount of effort into parenting) perhaps heavy taxes on child #3 onward (only on children conceived after the restriction was put in place, of course) would be reasonable.
And perhaps if more doctors/etc would promote the option of adoption and contraception, (instead of making it nearly impossible for a young person to get sterilized or even get an IUD like they do now) that would help a lot too. I personally can't see why someone WOULDN'T choose adoption. I mean, you get to pick out your own kid, and you don't have to suffer through 9 months of crap! Sounds like a fantastic deal to me. I wouldn't roll a die to choose a DOG, why would I take a chance like that on a kid?