Well, what the heck.
Lisa's point -- there'd be evidence -- is hard to dismiss. It's not like it's hard to measure radiation levels. Okay, maybe you can't go down to you local Walmart and pick up a geiger counter, but it's a little hard to believe that such a coverup is possible. Granted, it's not the same as underground nuclear tests which contaminate the soil and don't disperse, but it's hard to believe that a significant problem in the area wasn't seen and reported at the time if it was there. It's quite a bit easier to believe that a muckraking reporter dug up a bunch of anectodal evidence and a few crackpot dissenters to put toghether a story. (To be clear, I mean no disrespect toward muckrakers, anecdotes, or crackpots -- they're often what you need to break open a get to the truth when it's hidden by either laziness or a bona fide conspiracy.) Problems regarding handling of radioactive material have hardly been underreported in general in the U.S. There are plenty of stories about poor conditions of mines, the damage from nuclear testing, the utter disaster of the Rocky Flats plant, and, yes, TMI.
So I don't dismiss the possibility that there was a serious impact out of hand, but I'm quite doubtful.
But there is another side to the story: What are the alternatives? Nuclear power might have some health and safety impacts. Coal incontravertably does, and that's where we get most of our power today. Thankfully mines in America aren't as bad as they used to be and are today in China where about 5000 people die every year in coal mining accidents, but they're still dangerous. Mining still kills. And so does the pollution from mines. And so does the effect of the CO2 released when you burn it. Nuclear power is hardly a silver bullet (well, fission anyway -- workable fusion power would be a real breakthrough, but that's not going to be around for decades, if ever). So here's the question:
Given the risks and costs involved in every single type of power generation available today, which is the best choice? Regardless what happened at TMI, that's not an easy question to answer, and the correct answer varies strongly with personal values.