The rice genome was sequenced because of it ’s major importance in the world food industry and it ’s relatively small genome size. Since the first sequenced plant genome (Arabidopsis thalia) was a dicotylous plant it made sense to sequence a monocot next to have a broad taxonomix sampling. Unlike many of the recently sequenced genomes the sequencing has been done using a BAC-by-BAC approach a very costly a labour intensive procedure. But this yields a very high quality genome that can be assembled into chromosomes.

Further reading:

The TIGR Rice Genome Annotation Resource: improvements and new features. Nucleic Acids Res 35, D883-887 (2007). [pubmed]