daknetworks.com

First get the current ALLOWED/BLOCKED state of the devices for an account"
Get-CASMailbox -Identity foo.user | Select-Object "*ActiveSync*"

Next get the device id for an account:
Get-MobileDevice -Mailbox foo.user| fl identity,deviceid

Allow the devices for the account:
Set-CASMailbox -Identity foo.user -ActiveSyncAllowedDeviceIDs @{add='D17VGSSFTP7VB799BVU1HTNH68'}
Set-CASMailbox -Identity foo.user -ActiveSyncAllowedDeviceIDs @{add='7044450f9e54415d907e56e5c09d517f'}