I posted about this on the community message board to see if anyone else received a note like we did and, sure enough, others have.
Maarten was correct in that somebody is out there fishing for people who might be having some financial trouble and contemplating selling their house. Our area hasn't been hit too hard by the mortgage crunch, but I guess there are enough people in trouble to attract the vultures.
Hopefully they just keep circling and eventually move on to some other place.
...or fly into the power lines.