Die-shrink of the Prescott core (90nm). Two advantages are: cheaper to manufacture for Intel due the smaller die-size and lower power consumption plus higher clock frequencies.
The Cedar Mill core is used in both the Celeron D as the Pentium 4 processors. These processors run fairly cool and can reach high clock frequencies. Overclockers have used Cedar Mill based Celeron D's to overclock them over 8GHz.
Intel released the Cedar Mill in early 2006; six months before the release of the Core 2 Duo. Originally Cedar Mill wouldn't have been the successor of Prescott. A processor with codename 'Tejas' would have been but was cancelled because of mediocre performance and a very high heat output. Rumors are that Tejas was cancelled just weeks before the first A0-samples of Tejas would have been manufactured.