Depends on the licensing/certification of the device - for consumer equipment (particularly scanners) it's required, for certain experimental and test equipment it is not.

EDIT: I think this happened in 1994 - google "cell blocked" scanner and you'll find stuff on it.

Edit Again: Here's a QRZ thread on it: http://forums.qrz.com/index.php?threads/why-are-ham-radios-s...