It has nothing to do with power.. I already posted a link that showed for the most part a 1.8 pentium M is about the same as a 2.8 P4.. that is still pretty powerful! now the bus speeds are not the best but that is to conserve battery life.. And the Moble A64s still dont have a great speed step solution like the pentium Ms do..
http://www.pcworld.com/news/article/0,aid,119366,00.asp