I've finally gotten around to playing with WSPR. i've been transmitting on the 40m frequency for a few minutes now, looking at reports of my transmissions.
Here's a plot of where my station is being heard, circa 2000z:

This looks very reasonable for NVIS reception. I am transmitting 5 watts. Of course WSPR is an excellent low signal level mode, so it would not be likely that a USB or AM transmission would be heard over this entire area, but the general range is as expected.
Quick summary of WSPR: Stations transmit during two minute slots, skipping now and then, and each station is [usually] on a slightly different frequency. Receiving stations decode transmissions and find as many as they can, and send this data to a website, where it is plotted. In addition to transmitting, you can look at the map and list of reports to see who is being heard where. As always, you are somewhat limited by how many stations are actually listening, and where they are located. It's a great way to observe real time propagation on the various ham bands.