Thanks, Victoria. It seems like it might be a helpful resource. I'll report back in a week to see if sales rankings improved for the authors who had titles listed.
ETA: Okay, it's May 20th. I've followed a few of the authors who advertised with Kindle Nation and found nominal results. A couple authors were ranking in the 20,000 range and were still doing so after six days. Others were ranking in the 100,000 range and are still there. It didn't seem to draw sales, although it didn't detract either. It seems to have maintained the status quo.
My little sampling is by no means exhaustive and I did not follow all authors who paid for advertising, so make of it what you will. The lowest priced option was around $25, IIRC. It might be worth trying, but I wouldn't bet the farm on it. I think money would be better spent in other areas (editing, cover art, expanded distribution, etc.).