I just picked up the sony google tv last night and was frustrated that I couldn't figure out how to turn off my directv box from the remote. Coming from the revue I am used to being able to turn on/off everything from the remote.
I figured out how to do it even though you have to go through some menu's, but at least it can be done.
On the remote
1. Press Menu (down on the right thumb pad)
2. Select Advanced Options
3. Select Power On/Off
That should turn your cable box on or off.