6 answers
I don't believe that you can do this within the report, but you can do it with a chart and a dashboard.
Simply add a chart to your report. You can then customize the results to display the results in decending sort order.
This can also be done on a Dashboard as a table. You'll need to have a chart on the report to do this. (Why? Cause.) Click the wrench, click the customize the chart hypertext and select, Value, decending.