Ars Technica speculates that this is another reason that Apple decided to leave Flash off MacBook Airs (instead of just to make sure that users would download the latest version). It's important to note that Ars Technica isn't saying simply having the Flash plug-in installed on the Air reduces battery time, but that when visiting the same websites in Safari with Flash installed and then with Flash uninstalled, Flash ads became static ads, thus resulting in the higher battery life. In the end, Ars found that "Flash-based ads kept the CPU running far more than seemed necessary."
While Ars only tested the Flash battery drainage on a MacBook Air, it seems logical to assume that similar battery runtime results would be seen on other MacBooks and PC laptops. I'm about to test this out myself by removing Flash from my 15-inch MacBook Pro and seeing if my wireless web surfing time increases.
Besides uninstalling Flash completely from your machine, there are a number of Safari extensions that allow you to disable or enable Flash with a click.