Is willpower a finite resource?

This Psychology and Heath article, as quoted at Futurepundit, says maybe so:

"Cognitive tasks, as well as emotional tasks such as regulating your emotions, can deplete your self-regulatory capacity to exercise," says Kathleen Martin Ginis, associate professor of kinesiology at McMaster University, and lead author of the study.

Martin Ginis and her colleague Steven Bray used a Stroop test to deplete the self-regulatory capacity of volunteers in the study. (A Stroop test consists of words associated with colours but printed in a different colour. For example, "red" is printed in blue ink.) Subjects were asked to say the colour on the screen, trying to resist the temptation to blurt out the printed word instead of the colour itself.

"After we used this cognitive task to deplete participants' self-regulatory capacity, they didn't exercise as hard as participants who had not performed the task. The more people "dogged it" after the cognitive task, the more likely they were to skip their exercise sessions over the next 8 weeks. "You only have so much willpower."