What makes the Atrix's implementation of Citrix Receiver special? The Motorola Atrix looks like a nice phone. 

A widely publicized feature is the remote desktop function, which uses Citrix Receiver to connect to a virtualized workstation in the cloud, so that one could potentially plug the phone into any TV and start working.

My question is: how much of this function is unique to the Atrix? It looks like Citrix Receiver can be used on any Android device, indeed it's available for other smartphone platforms as well. Other smartphones have HDMI output, so that is not a differentiating factor.

Could I replicate this function on any Android phone with an HDMI jack?
If you're asking if you can do remote desktop on a phone and also have the phone output to a TV, the answer's yes.  I use Remote Desktop Enterprise on my device (Samsung Galaxy S Vibrant), and my device supports some sort of output from the USB port to HDMI as well as wireless DLNA.