As I reported earlier, the booing started when he was first introduced. I have video evidence to back it up. (Sorry about the poor camerawork.)
The Daily Kos has posted links to other videos confirming my account. Salon and the New York Times agree that Cheney was booed before, during, and after the first pitch. And you can hear the booing in the Post's own video.
Unfortunately for Nats fans, the Post got one thing right: Unlike last year, there were no 7th inning heroics, and the Nats lost 7-1, with the only run coming off Alfonso Soriano's homer.