I've used VNC through USB-networking on N900 and also X-applications on N900 using PC as a console.
I am sure many have.

Using video-out with USB-mouse and keyboard is closer to that demo though.

But, yes it is pretty cool. HDMI-output is nice to have but I hope Nokia will put also the 3.5mm AV-output in future devices.

I wonder how easy that Atrix is inserted to that docking station so both HDMI and microUSB connectors plug in properly.