Hourly employees and off-site e-mail access: What are the wage-and-hour rules?

Q. Several of our hourly employees have requested access to their office e-mail from their iPhones, BlackBerrys and other similar devices. We are inclined to allow this access, but want the employees who receive access to sign express waivers to the effect that they will not be “on-the-clock” while doing so. Can we legally require such a waiver?