Intelligence is not the be-all end-all quality for which we should be selecting. In fact, there is no single such quality. Sorry charismatic people, and kind people, and risk seekers and people with anomalously good immune systems...but you have to pass this test to have babies, and charisma, kindness, risk tolerance and great fortitude get you zero points.
That aside, how would get the dumb to line up for the genocide? You are setting yourself up for revolution. Also, suppose you happen to fall in love with someone who's IQ measured slightly too low? Suppose the person was sick on the day of the test? Who administers these tests? How many trillions of dollars are we going to spend to make sure the test is accurate and fair, then to fight the coming War of the Dummies, and then to sterilize the survivors? (Remember, according to John Kerry, the U.S. military is mostly made up of the ones you want to sterilize.)
Besides, let's say that only the top 50% gets to procreate and that society becomes smarter as a whole without any negative genetic consequences that normally come with a thinning of the gene pool, who drives the buses in your world? Who picks up the garbage? Who wades through the sewage when it backs up into the street? The smarter you are, the less likely you'll be satisfied in some menial job.
I never thought I'd say it, but Huxley's Brave New World is a better solution than this, as at least he thought through those little problems.