Policy for Employee Personal Devices (USB HD’s, PDA’s, etc.)

We are trying to come up with a policy (stance) on the use of personal devices in our company. Our company has always put user functionality ahead of security, but we would like to find some middle ground and wondered what everyone else was doing about users bringing in 250GB USB hardrives, personal phone/PDA’s that synch email/files, and flash drives.

Thanks,
Jonathan