This TechNet article on Windows Phone 7 got my attention. It appears you cannot fully utilize Exchange ActiveSync mailbox policies, unless you set AllowNonProvisionableDevices to True. If you don’t do that, you can only use the following properties, otherwise synchronization issues might be experienced:

• PasswordRequired
• MinPasswordLength
• IdleTimeoutFrequencyValue
• DeviceWipeThreshold
• AllowSimplePassword
• PasswordExpiration
• PasswordHistory
• DisableRemovableStorage
• DisableIrDA
• DisableDesktopSync
• BlockRemoteDesktop
• BlockInternetSharing

Another option is to create a seperate policy for Windows Phone 7 users.

Another thing worth mentioning is that when using multiple Exchange accounts on your Windows Phone 7, policies will be merged into a most restrictive set (credit to Dave Stork who got the information at TEE10).