This scale-out approach was also recently mentioned by Tim O'Reilly's, MySQL: The Twelve Days of Scaleout.
Scale-up x Scale-out: A Case Study using Nutch/Lucene
The Nutch/Lucene search framework includes a parallel indexing operation written using the MapReduce programming model . MapReduce provides a convenient way of addressing an important (though limited) class of real-life commercial applications by hiding parallelism and fault-tolerance issues from the programmers, letting them focus on the problem domain.
The query engine part consists of one or more front-ends, and one or more back-ends. Each back-end is associated with a segment of the complete data set...The front-end collects the response from all the back-ends to produce a single list of the top
documents (typically 10 overall best matches).
We see that the peak performance of the scale out solution (BladeCenter) is approximately 4 times better.