In GATK 2.6, there have been some changes to BaseRecalibrator. Based on the AnalyzeCovariates page, it must now be run twice. To generate the first pass recalibration table file, it's the same command as before. To generate the second pass recalibration table file, you need to add the -BQSR argument. However, on the BaseRecalibrator page, there is no -BQSR documentation.