I would say that the FortyHourWeeks are good to the XP concept:
- Increases efficiency of employees
- Improved scheduling
- Better management
- Work-life balance
This does help for those companies who take up XP and don't know the importance of these points. I've been involved in various such small to medium sized companies who rarely do well because of sleepy employees. This point helps a lot but needs to be given more focus in discussions.