Here are my unscientific observations.
- idle with no devices except a NIC, wifi, bluetooth: 7 to 8 watts
- idle with no devices except a NIC: 7 to 8 watts -- that was unexpected
- CPU at ~80% scp'ing a large file to internal disk: 9 watts
- CPU at ~80% scp'ing a large file to external, powered eSATA disk: 9 watts
- idle with external unpowered 320G 'laptop' USB disk: 11 watts
Powertop didn't have much in the way of suggestions that affected power.