If you look at: http://comments.gmane.org/gmane.comp.jakarta.lucene.hadoop.user/30662, you will find that bzip2 format is indeed splittable and multiple mappers can work on one file. The patch was submitted at: https://issues.apache.org/jira/browse/HADOOP-4012. However, it seems it is available only above HADOOP 0.21.0.
From personal experience in order to use this technique of bzip2 there is nothing different that you need to do. hadoop should pick it up automatically depending on your min split size.
bzip2 compressed data by blocks and therefore it is possible to decompress it in blocks and send each block to a separate mapper. However, gzip does not have such a technique and therefore this cannot be sent to different mappers.