[QUOTE=Right Way Up;9615850]US ATC are not obligated to check readbacks and challenge.
ATC is definitely required to listen to and correct readbacks. If we for example issue a descent to FL 330 and it is acknowledged with a readback of "descend and maintain FL 230" we are now responsible to the same extent as if we had issued the descent to FL 230. On rare occasions I have seen the FAA try to also fault the flight crew, but without fail if a loss of separation of any sort occurs we will be considered at fault.