Actually, you dont need to replace the pipes. They can actually line them in place. Of course then people might have concerns about the chemical makeup of the liners (basically plastic), but it's hard to say what other alternatives are out there.

That statement has the exact same problem. With sufficiently sensitive tests you might in theory be able to detect the negative effects of a single lead atom.

What we really want to know is: how big of a deal is this really?

There are unlikely to be people in the US with blood levels less than 1 order of magnitude less than the EPA action threshold. Lead is in your drinking water, food, and if you lived near anything with leaded - your air.

We used to use lead based pesticides at the dawn of the 20th century, if you eat any food grown in the midwest you are exposing yourself to reasonably large quantities of lead.

Does a single atom have known negative effects?

In the US at least, were it structured as an infrastructure project on a city by city basis, it would actually be extremely possible to strip out lead pipes and replace them with something modern.

The problem is that pipe ownership in a domestic home is the property of the homeowner, and there's no political will to do that kind of massive wealth transfer from the public coffers to the improved infrastructure of millions of individual properties. But while it wouldn't be inexpensive, it's on par with other expenditures the federal government makes for federal projects.