Unlike Pentium D, which is based on Intel's Pentium 4 desktop processor, Yonah is based on their mobile archtecture, Pentium M, but provides dual core in a mobile package.  As a result, for laptops, as well as servers where power consumption is an issue (which is nearly all servers) Yonah offers significant performance benefits.  See the anandtech story, as well as others referencing it.
What is so exciting about Intel Yonah?