If a buy a computer with a CPU that is rated at X GHz then that CPU had better be able to maintain that frequency, always. Otherwise it's a meaningless number. CPUs can already overclock themselves (Turboboost) above that frequency so if they can also legitimately underclock themselves then the 'rated frequency' is completely meaningless. I don't think that is acceptable. I encourage all slashdot readers to test their new computers under load and if they cannot maintain their rated frequency RETURN THEM! Or better yet, file a formal complaint for false advertising or fraud and then return them.
I blogged about this a while ago and I think the problem has only gotten worse. Lots of consumers are getting a crap experience because of insufficient cooling, manufacturers are selling rigs that can't do what they promise, and software developers waste time dealing with complaints about slow games/etc.